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QUANTITATIVE    CHEMICAL   ANALYSIS. 


*  PRINTED     BY 

SPOTTISWOODE    AND    CO.,     NEW-STREET    SQUARE 
LONDON 


QUANTITATIVE 


CHEMICAL  ANALYSIS; 


BY 


T.  E.  THORPE,  PH.D.,  B.Sc.(ViCT.),  F.R.S. 

PROFESSOR   OF   CHEMISTRY   IN   THE   NORMAL  SCHOOLS   OF   SCIENCE,    SCIENCE   AND 
ART   DEPARTMENT,    SOUTH   KENSINGTON,    LONDON. 


NINTH  EDITION. 


NEW    YORK: 

JOHN   WILEY   &   SONS,    15   ASTOR   PLACE. 
1891. 


PREFACE 

TO 

THE      FIFTH      EDITION. 


THE  PRESENT  EDITION  of  this  work  will  be  found  to 
include  a  considerable  amount  of  new  matter.  Many 
valuable  hints  and  suggestions  have  been  received 
from  teachers  and  others  both  in  this  country  and 
in  America.  Professor  Frankland  has  kindly  looked 
over  the  section  on  Water  Analysis ;  Professor  Dittmar 
has  fui  nished  an  account  of  his  method  for  the  valua* 
tion  of  chrome  ore ;  and  Mr.  Watson  Smith  has  sent 
the  results  of  his  experience  of  the  work  in  the  labora- 
tory of  the  Owens  College,  Manchester.  To  these 
and  to  other  gentlemen  who  have  furnished  him  with 
additions  and  corrections,  the  Author  begs  to  tender 
his  grateful  acknowledgments.  He  also  desires  to 
express  his  indebtedness  to  Mr.  C.  H.  Bothamley, 
Assistant-Lecturer  on  Chemistry  in  the  Yorkshire 
College,  for  aid  in  the  revision  of  the  book. 

YORKSHIRE  COLLEGE,  LEEDS  : 

237347 


PREFACE. 


THE  AIM  of  this  book  is  to  teach  the  principles  of 
Quantitative  Chemical  Analysis  by  the  aid  of  ex- 
amples chosen,  partly  on  account  of  their  practical 
utility,  and  partly  as  affording  illustration  of  the  more 
important  quantitative  separations. 

It  is  divided  into  five  distinct  parts.  The  first  part 
gives  a  description  of  the  balance,  of  the  mechanical 
principles  involved  in  its  construction,  and  of  the 
manner  of  using  it.  It  also  contains  an  account  of 
the  operations  generally  or  most  frequently  occurring 
in  Quantitative  Analysis  ;  such  as  the  process  of  fil- 
tration, the  incineration  of  filters,  and  so  forth. 

The  second  part  consists  of  a  graduated  series  of 
examples  in  simple  gravimetric  analysis,  commencing 
with  the  analysis  of  copper  sulphate,  and  ending  with 
the  estimation  of  arsenic,  antimony  and  tin. 

The  third  part  treats  of  volumetric  analysis.  The 
more  important  volumetric  processes  are  fully  de- 
scribed, and  their  application  is  illustrated  by  ex- 
amples of  scientific  as  well  as  of  technical  interest 


viii  Preface. 

The  fourth  part  contains  an  account  of  the  methods, 
gravimetric  and  volumetric,  employed  in  the  analysis 
or  valuation  of  ores,  minerals,  and  of  the  more  im- 
portant industrial  products,  such  %as  copper  and  lead 
ores,  iron  and  manganese  ores,  limestone,  cast  and 
wrought  iron,  soda-ash,  bleaching  powder,  &c.  Con- 
siderable space  has  been  allotted  to  the  important 
subject  of  water- analysis. 

The  fifth  part  treats  of  the  general  processes  of 
organic  analysis. 

The  Author's  thanks  are  due  to  his  assistant,  Mr. 
Dugald  Clerk,  for  the  attention  he  has  bestowed  on 
the  drawings  for  the  woodcuts.  The  illustrations  in 
the  section  on  *  Water-analysis '  are  taken,  with  Mr. 
Sutton's  kind  permission,  from  his  work  on  '  Volumetric 
Analysis/  In  the  account  of  Frankland  and  Arm- 
strong's method  of  determining  the  amount  of  organic 
carbon  and  nitrogen  in  water,  it  will  be  seen  how 
much  the  Author  is  indebted  to  Mr.  Wm.  Thorp's 
excellent  description  of  the  process  in  that  manual 
Mr.  Crookes  has  also  kindly  allowed  the  use  of  the 
figures  in  illustration  of  Luckow's  process  for  assay- 
ing copper-ores. 


CONTENTS. 


PART  I. 

PRINCIPLES  OF  QUANTITATIVE  ANALYSIS. 

PAGE 

The  Balance I 

The  Weights .     16 

The  Operation  of  Weighing     .......         20 

General  Preliminary  Operations  .         .         .         .         .  -33 


PART  II. 


SIMPLE  GRAVIMETRIC  ANALYSIS. 

I.   Copper  Sulphate  .         .         .         .         .         .         .  70 

II.   Sodium  Chloride      .......  79 

III.  Pearl-ash 83 

IV.  Rochelle  Salt 85 

V.   Dolomite      . 85 

VI.   Barium  Sulphate       .         .         .         .         .         .         -91 

VII.   Indirect  Estimation  of  Barium  and  Calcium    .         .  92 


Contents. 

PAGE 

VIII.  Ferrous  Ammonium  Sulphate  .    -. ,  .      ".         .         -94 
IX.  Determination  of  Nitric  Acid        ....         96 

X.  Potash-alum    .         .  / 98 

XL  Glass  .         .         . 99 

XII.   Felspar 101 

XIII.  Brass,  Bronze,  Gun-Metal,  Bell-Metal  .         .         .103 

XIV.  German  Silver         .  ~   %  .         .         ,         .         .         .   106 
XV.   Britannia  Metal 107 

XVI.  Type-Metal .         .         .         .108 

XVII.  Fusible  Metal  ...       109 


PART  III. 

SIMPLE  VOLUMETRIC  ANAL  YSIS  OF  SOLIDS  AND 
LIQUIDS. 

I.  Determination  of  Chlorine  by  Standard  Silver  Solu- 
tions        .         .         .         .         .         .         .         .124 

II.  Indirect  Determination  of  Potassium  and  Sodium 
by  means  of  Standard  Silver  Solution  and  Potas- 
sium Chromate  .  .  .  .  .  .  .128 

III.  Estimation  of  Chloric  Acid  .....        129 
Ilia.  Determination  of  Chlorine  in  presence  of  Sulphites   .   129 

ALKALIMETRY. 

IV.  Valuation  of  Soda- Ash          ....        v      137 
V.  Estimation  of  Alkaline  Hydrate  in  presence  of  Car- 
bonate     ,'.      ..         .      ,   .    «     .';  ..'•  .         .         -138 

VI.  Estimation  of  Sodium  Carbonate  in  presence  of  Potas- 
sium Carbonate         .         .       -.'T   ,-.       ,.,    .  .        T39 
VII.  Determination  of  Ammonia      .         ,         .       ',.--,,.    140 


Contents.  xi 


ACIDIMETRY. 

PAGE 

VIII.   Determination  of  the  Strength  of  Acetic  Acid,  Pyro- 

ligneous  Acid,  Vinegar  .         .         .         .         .         .142 

IX.   Determination  of  combined  Carbon  Dioxide    .         .        143 
X.   Estimation  of  Carbonic  Acid  in  Natural  Waters          .    144 
XI.   Estimation  of  Carbon  Dioxide  in  Artificially  Aerated 

Waters 145 

XII.   Determination  of  Combined  Acids  in  Salts          .         .   146 


ANALYSIS  BY  OXIDATION  AND  REDUCTION. 

XIII.  Determination  of  the  Strength  of  the  Permanganate 

Solution     ........        148 

XIV.  Volumetric  Estimation  of  Calcium  by  means  of  Potas- 

sium Permanganate         .         .         .         .          .         -152 
XV.  Volumetric  Estimation  of  Lead  by  Permanganate  Solu- 
tion    153 

XVI.  Valuation  of  Manganese  Ores  by  means  of  Potassium 

Permanganate  Solution  .         .         .         .         .         -154 

XVII.   Estimation  of  Potassium  Ferrocyanide  by  Permangan- 
ate Solution        .         .         .         .         .          .         .154 

XVIII.   Estimation  of  Potassium  Ferricyanide  by  Permangan- 
ate Solution  .         .         .  .         .         .         -154 

XIX.   Valuation  of  Bleaching-powder  by  Iodine  and  Sodium 

Thiosulphate  Solutions        .         .         .         .          .158 

XX.   Estimation  of  the  Amount  of  Chlorine  in  Aqueous 

Solutions  of  the  Gas        .         .         .         .         .         .158 

XXI.  Estimation   of  the   Amount   of  Sulphur   Dioxide  in 

Aqueous  Solutions  of  the  Gas      .         .         .  159 

XXII.   Estimation  of  Sulphuretted  Hydrogen  in  Aqueous  So- 
lutions of  the  Gas  .         .         .         .         .         .         -159 

XXIII.  Estimation  of  Hydrocyanic  Acid  .         .        160 

XXIV.  Estimation  of  Antimony  in  Tartar  Emetic  .         .161 
XXV.  Determination  of  Tin  by  Iodine  Solution         «         .        162 


xii  Contents. 


ANALYSES  BY  MEANS  OF  IODINE  AND  SODIUM  THIO- 
SULPHATE  SOLUTIONS,  WITH  PREVIOUS  DISTILLA- 
TION WITH  HYDROCHLORIC  ACID. 

PAGE 

XXVI.   Analysis  of  Potassium  Bichromate      ....    163 

XXVII.   Estimation  of  Arsenious  Acid        ....        165 

XXVIII.   Analysis  of  Chlorates,  Bromates,  and  lodates     .         -165 
XXIX.   Estimation  of  Iron  by  means  of  Iodine  and  Thiosul- 

phate  Solutions  .          .         .          .          .          .         .166 

XXX.  Estimation  of  Nitric  Acid  by  Solutions  of  Iron,  Iodine, 

and  Sodium  Thiosulphate        .         .         .          .         .167 

XXXI.  Valuation  of  Manganese  Ores  by  Distillation  with 
Hydrochloric  Acid,  and  Titration  with  Iodine  and 
Thiosulphate  Solutions  .  .  .  .  .167 


PART  IV. 

GENERAL  ANALYSIS,  INVOLVING  GRAVIMETRIC  AND 
VOLUMETRIC  PROCESSES. 

I.   Nitre        .         ...     .'        .     '    .  v        .168 

II.   Gunpowder  .         .         .         .        •''.         .         .         .        17° 

III.  Limestones.     Hydraulic  Mortar        .         .         .         .    174 

IV.  Clays   .         .         .      'f  .       '  V     '  .    •    • .  '    '  .      i  .       180 
V.  Assay  of  Manganese  Ores  (Pyrolusite,  Braunite,  &c. ) 

Gravimetrical  Method  of  Fresenius  and  Will         .        185 
VI.  Volumetric  Determination  by  means  of  Iron  and  Po- 
tassium Permanganate  Solution        .        •'.'        .         .189 
VII.   Volumetric   Determination  by  means  of  Oxalic  Acid 

and  Potassium  Permanganate       .          .          .         .189 
VIII.  Determination  of  Moisture  in  Manganese  Ores   .  190 


Contents.  xiii 

PAGE 

IX.   Determination  of  the  Amount  of  Hydrochloric  Acid 

required  to  decompose  Manganese  Ore          .         .  igi 

X.   Bleaching  Powder     .......  193 

XI.   Black- Ash  ;  Soda-Ash  ;  Vat-Waste        ...  198 
XII.   Estimation  of  Sulphur  in  Pyrites,  by  means  of  Copper 

Oxide    .........  205 

XIII.  Assay  of  Copper  Ores  (Mansfeld  Process)        .          .  205 

XIV.  Assay  of  Copper  Ores  (Luckow's  Process) .         .         .  207 
XV.   Assay  of  Copper  Ores  by  Precipitating  the  Metal  by 

means  of  Zinc     .......        2IO 

XVI.  Copper  Pyrites          .         .         .         .         .          ,         -211 

XVII.   Iron  Pyrites 215 

XVIII.    «  Kupfernickelstein ' 215 

XIX.   Iron-Ores 215 

XX.  Titaniferous  Iron-Ore  (Ilmenite)         ....    225 

XXI.  Wrought  and  Cast  Iron  and  Steel  ....        226 

XXII.  Iron  Slags .'  249 

XXIII.  Assay  of  Zinc- Ores         ......       250 

XXIV.  Assay  of  Tin-Ores    .......   252 

XXV.  Separation  of  Tin  from  Tungsten    .         .         .         -253 

XXVI.   Wolfram 2^4 

XXVII.   Scheelite 255 

XXVIII.   Galena 255 

XXIX.   Refined  Lead 258 

XXX.   White  Lead     .         .    ' 267 

XXXI.   Chrome  Iron-Ore 268 

XXXII.    Smaltine  :  Cobalt-Glance 272 

XXXIII.  Fahl-Ore  (Tetrahedrite) 276 

XXXIV.  Determination  of  Silver  in  Solutions  ....  279 
XXXV.   Assay  of  Silver  in  Bullion,  Coin,  Plate,  &c.     .         .281 

XXXVI.  Assay  of  Gold .  286 

XXXVII.   Separation  of  Gold,  Silver,  and  Copper  .         .       286 

XXXVIII.   Estimation  of  Mercury 287 

XXXIX.   Coal 289 

XL.  Examination  of  Water  used  for  Economic  and  Techni- 
cal Purposes    ....  .  291 


xiv  Contents. 

PAGE 

XLI.  Determination  of  the  Amount  and  Nature  of  the  Gases 

dissolved  in  Water         .'',.'        .      "  ..  '   .         .  327 
XLII.  Guano        .         .    ~"  V     •  .  ,x      .  ,  \  . '   ,  ''  .         .         331 

XLTII.  Bone-Dust       .  337 

XLIV.    Superphosphates 337 

XLV.  Ashes  of  Plants        .......  340 


PART  V. 


ORGANIC  ANALYSIS. 

I.  Analysis  of  Bodies  containing  Carbon  and  Hydrogen, 

or  Carbon,  Hydrogen,  and  Oxygen          .         .          .   348 
II.   Analysis  of  Organic  Substances  containing  Nitrogen        358 

III.  Analysis  of  Organic  Substances  containing  Chlorine. 

Bromine,  and  Iodine     "    .         .         .         ,        |r        364 

IV.  Analysis  of  Organic   Substances  containing  Sulphur 

and  Phosphorus    .         .        \         .         .         .         .  366 


APPENDIX    .         „         .-.,,,.         ,        V     "'  -         •         -        369 


QUANTITATIVE 

CHEMICAL     ANALYSIS. 

PART     I. 
PRINCIPLES    OF     QUANTITATIVE    ANALYSIS. 

THE  BALANCE.   GENERAL  PRELIMINARY  OPERATIONS. 

QUANTITATIVE  ANALYSIS  is  that  branch  of  Practical 
Chemistry  which  treats  of  the  processes  by  which  we  deter- 
mine the  relative  amounts  of  the  constituents  of  a  body. 
QUALITATIVE  ANALYSIS  informs  us  simply  of  the  nature  of 
these  constituents,  and  teaches  us  how  they  may  be  sepa- 
rated. The  latter  form  of  analysis  always  precedes  the 
former,  for,  obviously,  we  must  first  know  what  are  the 
elements  present  in  a  substance  before  we  can  proceed  to 
estimate  their  proportions. 

The  methods  of  Quantitative  Analysis  are  subdivided 
under  the  two  heads  of  Gravimetric  Analysis  and  Volumetric 
Analysis.  By  means  of  Gravimetric  Analysis  we  seek  to 
weigh  the  known  constituents  of  a  substance  either  in  their 


2          „        Quantitative  Chemical  Analysis. 

elementary  condition,'  or  in 'the  form  of  combinations  which 
admit  of  exact  weighing,  and  of  which  the  composition  is 
already  accurately  known.  Supposing  that  we  wish  to  deter- 
mine the  composition  by  weight  of  a  sixpenny  piece  :  quali- 
tative analysis  tells  us  that  the  coin  is  made  up  of  silver  and 
copper.  We  may  determine  the  proportion  of  the  two  metals 
in  the  solution  of  the  coin,  either  by  separating  them  out  and 
weighing  them  in  their  metallic  state,  or  we  may  convert  the 
silver  into  silver  chloride,  and  the  copper  into  cupric  oxide, 
and  weigh  the  two  compounds.  Since  we  know  the  com- 
position of  the  silver  chloride  and  cupric  oxide,  we  can 
calculate  the  amount  of  silver  and  copper  respectively  con- 
tained in  them,  and  in  this  manner  determine  the  relative 
quantities  of  the  metals  present  in  the  coin.  In  practice,  it 
is  usually  found  more  convenient  to  estimate  the  constitu- 
ents in  a  body  by  the  aid  of  combinations  of  known  com- 
position, rather  than  to  attempt  to  isolate  the  elements.  It 
is  evident,  therefore,  that  a  correct  knowledge  of  the  pro- 
portion in  which  the  several  elements  are  present  in  these 
fiduciary  combinations  is  of  the  highest  value  to  the  analyti- 
cal chemist ;  and,  further,  that  the  exact  determination  of 
the  combining  weights  of  the  elements  becomes  to  him  a 
matter  of  primary  importance. 

But  it  will  be  obvious  on  reflection  that  we  can  deter- 
mine the  quantitative  composition  of  the  sixpenny  piece 
without  directly  weighing  either  the  metals,  or  their  com- 
binations with  chlorine  and  oxygen.  We  might  determine 
the  amount  of  the  silver,  for  example,  by  ascertaining  the 
quantity  of  hydrochloric  acid  required  to  convert  it  com- 
pletely into  silver  chloride.  We  know  that  if  we  add  hydro- 
chloric acid  to  solution  of  silver  in  nitric  acid  (silver  nitrate) 
we  obtain  insoluble  silver  chloride  ;  and  that  if  we  add  a 
sufficiency  of  hydrochloric  acid  the  whole  of  the  silver  will 
be  thrown  out  of  solution  : 

AgNO3  +  HC1  =  AgCl  +  HN03. 


The  Balance.  3 

Now,  if  we  know  how  much   hydrochloric  acid   (HC1)  is 
contained  in  any  given  volume  of  the  solution  which  we 
employ  to  precipitate  the  silver,  and  if  we  have  the  means 
of  recognising  the  exact   point  at  which  the  formation  of 
silver  chloride  ceases,  we  can  calculate  from  the  volume  of 
acid  required  the  amount  of  the  silver,  since,  as  the  equation 
tells  us,  36*46  parts  of  hydrochloric  acid  are  equivalent  to 
107*93  parts  of  silver.      This  is  the  fundamental  principle  o* 
volumetric  analysis,  a  form  of  quantitative  analysis  in  which 
we  seek  to  estimate  the  amount  of  a  substance  from  the 
determinate    action    of    reagents    in   solutions   of   known 
strength,  the  amount  of  the  reacting  substance  being  calcu- 
lated from  the  volume  of  liquid  used.      Many  examples 
might  be  adduced  to  show  the  wide  applicability  of  this 
principle  of  analysis.    Let  us  suppose  that  we  wish  to  deter- 
mine the  amount  of  sodium  hydrate  in  an  aqueous  solution 
of  this  substance.     If  we  add  a  few  drops  of  litmus  tincture 
to  the  liquid  we  obtain  a  blue  colouration,  which,  on  the  con- 
tinued addition  of  hydrochloric  acid,   eventually  becomes 
permanently  red.     The  acid  combines  with   the  alkali  to 
form  common  salt,  which  is  without  action  on  the  colour  of 
litmus  ;  the  final  change  in  colour  shows  us  that  the  whole 
of  the  sodium  hydrate  is  in  combination,  and  that  the  acid 
is  in  very  slight  excess.     If  the  strength  of  our  hydrochloric 
acid  solution  is  known  to  us,  that  is,  if  we  can  say  how 
many  grams  of  H  Cl  are  contained  in  1,000  cubic  centimetres 
(for  instance)  of   the   liquid,  we  can  calculate,  from   the 
number  of  cubic  centimetres  we  require  to  add  to  the  soda 
solution  coloured  with  litmus  before  it  is  permanently  red- 
dened, how  much  sodium  hydrate  is  contained  in  the  alka- 
line   liquid    originally  taken,    from    the    knowledge    that 
HCl=NaHO ;  i.e.  that  36*46  grams  of  hydrochloric  acid  are 
equivalent  to  40*04  grams  of  sodium  hydrate. 

THE    BALANCE. 

The  balance  affords  the  only  practicable  means  of  mea- 
B  2 


4  Quantitative  Chemical  Analysis. 

suring  the  mass  of  the  various  forms  of  matter  contained  in 
a  substance.  Practically  speaking,  this  instrument  consists 
of  an  inflexible  metallic  lever  or  beam  suspended  near  its 
centre  of  gravity  on  a  fulcrum  or  pivot,  the  masses  to  be 


compared  being  also  suspended  from  pivots  placed  at  the 
extremities  of  the  beam,  equidistant  from,  and  in  the  same 
horizontal  line  with,  the  central  fulcrum.  For  a  complete 
treatment  of  the  mechanical  theory  of  the  balance  we 


The  Balance. 


must  refer  to  special  treatises  on  the  subject  :  in  this 
work  we  shall  mainly  confine  ourselves  to  the  essential 
points  in  its  construction  and  mode  of  use,  and  only  touch 
on  the  mechanical  problem  in  so  far  as  it  appears  necessary 
to  enable  the  student  to  understand  the  conditions  of  sen- 
sibility and  accuracy  in  the  instrument.  Fig.  i  gives  a 
representation  of  a  modern  chemical  balance.  The  beam 
a  a  has  the  shape  of  an 
acute  rhomboid ;  this 
form  of  construction 
combines  lightness 
with  inflexibility  and 
strength  :  on  the  pos- 


FlG.  2. 


FIG.  3. 


session  of  these  quali- 
ties in  the  beam  much 
of  the  sensibility  and 
accuracy  of  the  balance 
depends.  Through  the 
centre  of  the  beam  passes  a  triangular  piece  of  hardened 
steel  or  agate,  termed  a  knife-edge,  the  lower  edge  of  which 
turns  upon  a  horizontal  plate  of  polished  agate  connected 
with  the  pillar.  At  the 
end  of  each  arm  is  a 
similar  knife-edge  fixed 
in  the  reverse  position, 
and  bearing  an  agate 
plate  from  which  de- 
pend steel  hooks  to  hold 
the  wires  attached  to  the 
pans  (fig.  2).  These 
terminal  knife-edges  are 
fixed  in  brass  settings, 
and  admit  of  being  ad- 
justed so  as  to  bring  them  into  exactly  the  same  plane  with 
the  centre  edge.  Their  relations  to  the  middle  knife-edge 
may  be  altered  by  means  of  the  little  screws  shown  in  the 


6  Quantitative  Chemical  Analysis. 

figure.  Various  other  methods  of  arranging  the  terminal 
knife-edges  and  pan-suspensions  have  been  proposed.  Fig.  3 
represents  a  form  adopted  by  continental  balance- makers. 

As  the  efficacy  of  the  instrument  depends  to  a  large 
extent  on  the  preservation  of  the  sharpness  of  the  knife- 
edges  and  the  smoothness  of  the  agate  planes,  it  is  desirable 
to  prevent  their  contact  when  the  balance  is  not  in  use. 
This  is  effected  by  means  of  the  frame  b  b  (fig.  i),  which 
lifts  the  centre  knife-edge  about  0*2  millimetre  from  the 
centre  plane  :  at  the  extremities  of  the  frame  are  steel 
points  which  enter  into  little  hollows  in  the  lower  surface  of 
the  pan-suspensions,  and  raise  them  from  the  terminal 
knife-edges.  This  frame  is  attached  to  a  rod  descending 
through  the  pillar,  and  connected  with  an  eccentric  worked 
by  a  milled-head  screw  (s)  situated  on  the  outside  of  the 
balance-case  :  by  means  of  this  movement,  the  rod,  and 
with  it  the  frame,  can  be  raised  or  lowered  at  pleasure.  In 
balances  of  the  highest  class  there  is  a  second  eccentric  con- 
nected with  a  system  of  bent  levers  which  carry  supports  for 
the  pans  ;  by  means  of  these  supports  the  pans  can  be 
steadied  whilst  the  weights  are  being  transferred,  or  their 
vibrations  can  be  checked  preparatory  to  releasing  the 
beam.  In  some  balances  the  pan-supports  are  worked  by 
an  independent  screw :  in,  others  they  are  worked  in  con- 
junction with  the  movement  which  raises  or  lowers  the 
frame.  Where  all  the  movements  are  controlled  by  a  single 
screw  these  are  not  made  to  act  quite  simultaneously. 
When  the  balance  is  to  be  set  in  vibration,  the  first  action 
of  the  screw  lowers  the  pan-supports  ;  it  next  brings  down 
the  centre  knife-edge  upon  the  agate  plane,  and  gradually 
allows  the  pan-suspensions  to  drop  simultaneously  upon  the 
terminal  knife-edges.  For  the  proper  performance  of  these 
movements  great  nicety  of  workmanship  is  needed,  for  it  is 
not  only  requisite  that  the  beam  and  pan-suspensions  should 
be  properly  raised  when  wanted,  but  it  is  also  necessary 
that  the  edges  and  planes  should  be  brought  into  contact  in 


The  Balance.  j 

a  constant  position.  The  movements  of  the  beam  are  in- 
dicated by  a  vertical  pointer  which  oscillates  before  an 
ivory  scale  fixed  to  the  pillar;  this  ivory  scale  is  usually 
graduated  into  20  parts,  and  its  middle  point  or  zero  is 
exactly  behind  the  needle  when  the  beam  is  horizontal.  Any 
inequality  in  the  weight  of  the  arms  is  compensated  by 
means  of  a  small  vane  fixed  on  the  top  of  the  beam  above 
the  central  knife-edge,  which  may  be  turned  to  the  right  or 
left  as  occasion  requires.  In  some  balances  this  compen- 
sation is  effected  by  means  of  little  screws  travelling  along 
fine  threads  attached  to  the  ends  of  the  beam  (see  fig.  3). 
The  stability  of  the  beam  is  regulated  by  the  aid  of  a 
weight  termed  the  gravity-bob  (g)  moving  along  the  rod 
attached  to  the  upper  edge  of  the  beam  over  the  centre  knife- 
edge  on  which  the  vane  works. 

In  order  to  protect  the  instrument  from  the  fumes  of  the 
laboratory,  and  to  prevent  air-currents  from  interfering  with 
its  action  during  the  operation  of  weighing,  it  is  enclosed  in 
a  glass  case,  the  back,  front,  and  sides  of  which  can  be 
opened  at  will.  The  case  is  supported  on  levelling  screws, 
by  which  it  can  be  adjusted  to  horizontality  in  accordance 
with  the  indications  of  a  spirit-level  or  plumb-line  attached 
to  the  instrument.  When  an  object  too  bulky  to  be  brought 
within  the  balance-case  has  to  be  weighed,  on  releasing  the 
screws  at  the  base,  the  pillar  and  beam  can  be  turned 
through  an  angle  of  about  60°,  so  that  the  ends  of  the  beam 
project  beyond  the  back  and  front  of  the  case.  The  proper 
adjustment  of  the  beam  on  the  part  of  the  balance-maker  is 
an  operation  of  the  greatest  nicety.  To  ascertain  if  the 
three  knife-edges  are  in  the  same  plane,  he  first  poises  the 
beam  without  weights  on  the  pans,  and  moves  the  gravity- 
bob  until  the  vibrations,  as  indicated  by  the  pointer,  become 
very  slow  •  he  then  puts  equivalent  weights  into  the  pans, 
and  again  sets  the  beam  vibrating  :  if  its  rate  of  vibration  is 
unaltered,  the  adjustment  is  perfect.  If  the  beam  vibrates 
too  quickly  or  oversets,  the  gravity-bob  is  raised  or  lowered 


8  Quantitative  Chemical  A  nalysis. 

so  as  to  bring  the  vibrations  to  the  original  rate :  the 
number  of  turns  required  to  effect  this  is  noted,  and  then 
the  bob  is  turned  in  the  contrary  way  through  double  the 
number  of  revolutions,  and  the  slow  motion  is  again  pro- 
duced by  means  of  the  adjustments  at  the  ends  of  the 
beam.  To  determine  whether  the  terminal  knife-edges  are 
at  equal  distances  from  the  centre  edge,  the  beam  is  poised 
with  weights,  and  the  pans,  together  with  their  suspensions, 
are  changed  from  side  to  side.  If  the  equilibrium  is  un- 
disturbed, the  edges  are  properly  adjusted  ;  if,  however,  one 
side  appears  heavier  than  the  other,  a  small  piece  of  bent 
wire  termed  a  rider  (see  fig.  7)  is  placed  on  the  lighter  side, 
and  pushed  along  the  beam  until  the  equilibrium  is  again 
established  :  the  rider  is  now  pushed  along  half  way  towards 
the  centre  of  the  beam,  and  the  adjustment  made  at  one 
end.  The  knife-edges  may  be  known  to  be  parallel  by 
hanging  little  hooks  upon  them  and  equipoising  the  beam  : 
on  sliding  the  hooks  along  the  knife-edges,  equilibrium 
should  be  maintained.  The  student  will  better  appreciate 
the  skill  required  in  these  adjustments  when  we  treat  of 
the  circumstances,  other  than  those  due  to  imperfect  work- 
manship, which  modify  the  action  of  the  balance. 

We  will  next  briefly  state  the  main  conditions  upon  which 
the  stability,  sensibility,  and  accuracy  of  the  instrument 
depend. 

i.  The  centre  of  gravity  of  the  balance  must  be  situated 
below  the  point  of  suspension,  i.e.  the  centre  knife-edge.  If 
the  balance  were  suspended  at  its  centre  of  gravity,  it  would 
be  in  the  condition  of  neutral  equilibrium,  and  the  beam 
being  once  disturbed  would  have  no  tendency  to  reassume 
horizontality,  but  would  remain  in  any  position  given  to  it  ; 
that  is  to  say,  the  beam  would  not  oscillate.  If,  on  the 
other  hand,  the  centre  of  gravity  is  above  the  point  of  sus- 
pension, the  beam  would  be  in  the  state  of  unstable  equi- 
librium, and  would  tend  to  overset  with  the  least  prepon- 
derating weight. 


The  Balance.  9 

2.  The  terminal  points  of  support  (knife-edges  for  pan- 
suspensions)  must  be  in  the  same  line  with  the  centre 
point  of  suspension.  In  other  words,  an  imaginary  straight 
line  drawn  from  one  terminal  edge  to  the  other  should 
just  touch  the  lowest  point  of  the  centre  knife-edge.  If 
the  centre  edge  is  below  the  line  joining  the  extreme 
points  of  suspension,  the  centre  of  gravity  of  the  whole 
system  will  be  raised  in  proportion  as  the  load  is  increased, 
and  at  a  certain  point  the  centre  of  gravity  will  be  exactly 
at  the  point  of  suspension  of  the  beam,  which  will  then 
cease  to  oscillate  ;  a  continued  increase  of  weight  will 
now  raise  the  centre  of  gravity  above  the  point  of  sup- 
port, when  the  beam  will  overset.  But  if  the  centre  edge 
is  situated  above  the  line  joining  the  extreme  edges,  the 
centre  of  gravity  of  the  whole  system  is  lowered  in  pro- 
portion to  the  increase  in  load  ;  that  is,  its  sensibility 
will  diminish  with  the  increase  of  weight  on  the  pans. 
When  the  three  edges  are  in  the  same  plane  an  increase 
of  weight  continually  tends  to  bring  the  centre  of  gravity 
nearer  the  point  of  support,  but  it  can  never  be  made 
to  coincide  with  it :  consequently  the  balance  will  never 
cease  to  vibrate.  We  thus  see  why  it  is  necessary  that 
the  beam  should  be  perfectly  inflexible.  If  the  weights 
acting  on  the  terminal  edges  caused  them  to  sink  below 
the  level  of  the  centre  edge,  the  centre  of  gravity  of  the 
whole  system  would  become  lowered,  and  the  sensibility 
be  lessened.  Theoretically,  increased  weight  creates  in- 
creased sensibility  in  the  instrument :  practically,  however, 
this  increase  in  sensibility  is  more  than  counterbalanced 
by  other  effects  of  increased  weight 

Let  a  be  the  central  knife-edge  supporting  a  beam  seen  end- 
on  (fig.  4),  and  b  and  c  the  terminal  points  of  suspension  ;  the 
straight  line  b  c  touches  the  point  a  of  the  centre  knife-edge. 
Fronitf  draw  the  vertical  a  d\  then  the  centre  of  gravity  of 
the  beam  must  be  somewhere  on  this  line  a  d :  let  us  suppose 
it  to  be  at  e.  If  equal  weights  w  are  suspended  from  b  and 


io  Quantitative  Chemical  Analysts. 


The  Balance.  1 1 

<r,  we  may  conceive  that  each  of  the  weights  acts  respectively 
at  b  and  c  :  their  common  centre  of  gravity  falls  at  # ,  and  the 
centre  of  gravity  of  the  whole  system  of  beam  and  load  is 
somewhere  between  a  and  e  along  the  vertical  a  d.  Since 
the  centre  of  gravity  remains  vertically  under  the  point 
of  support,  the  equilibrium  is  undisturbed.  Suppose  now 
that  an  additional  weight  w  is  made  to  act  at  cy  the  centre 
of  gravity  of  the  combined  load  is  no  longer  coincident 
with  a,  but  falls  somewhere  along  the  line  b  c  in  the  direction 
of  t,  say  at  f\  the  centre  of  gravity  of  the  whole  system  is 
at  some  point  along  the  line  e  /,  say  at  g.  The  centre  of 
gravity  is  no  longer  vertically  under  the  point  a :  accord- 
ingly the  beam  tends  to  revolve  until  this  condition  obtains 
(fig.  5).  The  arm  a  c  therefore  falls,  whilst,  of  course,  the 
arm  a  b  is  proportionately  raised.  The  angle  made  by  the 
beam  in  its  new  position  of  equilibrium  with  the  horizontal, 
due  to  the  preponderance  a/,  is  termed  the  angle  of  deviation : 
it  is  equal  to  the  angle  g  a  e.  This  angle  is  the  measure  of 
the  sensibility  of  the  instrument.  It  is  evident,  moreover, 
that  (3)  the  sensibility  of  the  balance  is  augmented  by  bringing 
the  centre  of  gravity  as  near  the  point  of  support  as  possible, 
whereby  g,  due  to  an  overplus,  becomes  proportionately 
raised  towards  the  line  b  c,  and  the  angle  of  deviation  g  a  e 
increased  in  the  direct  ratio  of  the  change.  If  e  is  so  far 
raised  as  to  be  identical  with  0,  then  on  the  addition  of  the 
weight  w,  g  would  fall  on  the  line  b  c  ;  that  is,  the  angle  of 
deviation  becomes  a  right  angle,  and  the  beam  consequently 
oversets.  The  distance  between  the  point  of  support  and 
centre  of  gravity  in  a  sensitive  instrument  probably  does  not 
exceed  TJF  of  a  millimetre.  The  arrangement  by  which 
the  alteration  in  the  centre  of  gravity  is  effected  has  already 
been  described.  There  is,  however,  a  limit  of  approxima- 
tion beyond  which,  in  ordinary  work  at  least,  it  is  practically 
inconvenient  to  go,  and  for  another  reason  than  that  just 
adduced.  As  the  centre  of  gravity  nears  the  point  of  sup- 
port, the  rapidity  of  the  vibrations  of  the  beam  diminishes, 


12  Quantitative  Chemical  Analysis. 

and  ultimately  becomes  very  slow  :  the  operation  of  weigh- 
ing may  thus  need  a  greater  expenditure  of  time  than  is 
warranted  by  the  degree  of  accuracy  required. 

4.  The  sensibility   of  the    instrument  increases   with   the 
length  of  the  arms.     If  ac  be  made  longer,  the  distance  af 
will  be  proportionately  increased,  and  the  point  g  will  be 
further  removed   from  the  vertical  a  d :    the  line  ag  will 
therefore  form  a  greater  angle  with  a  e  •  that  is,  the  angle 
of  deviation  will  be  increased.     Here,  too,  in  practice,  we 
quickly  find  the  limit  to  the  length  of  beam.     By  increasing 
the  length  of  the  beam  we  increase  its  weight,  and  by  in- 
creasing its  weight  we  diminish  its  sensibility. 

5.  The  sensibility  is  dependent  upon   the  lightness  of  the 
beam.     We  may  conceive  that  the  weight  2W  +  w,  acts  at 
the  point  ft  and  that  the  weight  x  of  the  beam  acts  at  e. 
The  position  of  the  centre  of  gravity  gt  along    the    line 
e  /,    is    obviously    dependent    upon   the    relation   of  the 
weights  acting  at  e  and/:    if  e  x  =  (2W  +  w)  /,   then  g 
will  be  equidistant  from  e  and  f,  and  the  smaller  x  becomes 
in  proportion  to  2W  +  w  the  further  will  g  be  removed  from 
<?,  and  therefore  the  greater  will  be  the  angle  of  deviation. 
With  the  view  of  increasing  the  sensibility  by  diminishing 
the  weight,  other  substances  than  brass  have  been  proposed 
as  the  material  of  balance-beams,  and  beams  have  actually 
been  constructed  of  aluminium,  which  possesses  a  specific 
gravity  of  only  2 '6,  less  than  one-third  that  of  brass. 

6.  The  sensibility  of  the  balance  is  also  affected  by  the  friction 
between  the  knife-edges  and  agate  planes.      The  immediate 
effect  of  this  friction  at   the  terminal  knife-edges  is  prac- 
tically to  vary  the  length  of  the  arms.       Let  us  suppose 
that  the  impediment  to  the  free  motion  of  the  planes  of  the 
pan-suspensions  is  at  its  maximum,  or,  in  other  words,  that 
the  planes  of  the  pans  are  maintained  parallel  to  the  direc- 
tion of  the  beam  during  the  oscillations  of  the  instrument. 
Such  an  instrument  would  be  perfectly  useless  as  a  balance, 
for  as  one  arm  was  depressed  by  the  action  of  a  prepon- 
derating weight,  the  heavier  pan  would  be  thrown  inwards, 


The  Balance.  1 3 

but  its  tendency  to  move  would  be  counterbalanced  by 
the  other  pan  being  thrown  correspondingly  outwards. 
This  variableness,  practically  speaking,  in  the  length  of  the 
arms  may  be  perceived  in  balances  of  which  the  edges 
have  become  blunted  by  wear.  Supposing  that  the  width 
of  the  knife-edges  is  x,  and  the  distance  between  their 
middle  points  is  jy,  a  glance  at  the  figure  (fig.  6)  enables  us 

FIG.  6. 


to  see  that  the  real  lengths  of  the  arms  must  be  x  —  y  and 
x  +  y.  Therefore  two  loads  possessing  the  ratios  of  x  -\-y 
and  x  —y  will  be  in  apparent  equilibrium.  It  is  said  that 
some  balances  will  indicate  one  part  in  a  million  ;  if  such  an 
instrument  possessed  a  20-inch  beam,  the  width  of  the 
knife-edges  cannot  exceed  ^nnnnro*  of  an  inch  (an  inappre- 
ciable amount  even  under  a  microscope),  and  the  two  arms 
must  be  adjusted  to  equality  within  this  length. 

When  properly  adjusted,  a  balance  should  satisfy  the  fol- 
lowing tests  : — i.  When  the  beam  is  released  the  pointer 
should  coincide  with  the  zero  on  the  scale,  or  make  slow 
excursions  tending  to  equality  of  amplitude  on  either  side. 
2.  The  equilibrium  should  not  be  disturbed  when  the  pans  are 
removed.  3.  If  possible,  the  position  of  the  beam  should  be 
reversed,  so  as  to  cause  the  arm  which  points  to  the  right  to 
point  to  the  left,  and  the  beam  again  be  made  to  oscillate  :  it 


14  Quantitative  Chemical  A  nalysis. 

should  vibrate  exactly  as  before,  and  finally  acquire  a  horizon- 
tal position.  Imperfect  workmanship  in  the  middle  knife-edge 
or  in  the  planes  on  which  it  works  is  immediately  indicated 
if  the  beam  now  behaves  differently.  In  a  good  balance  the 
centre  knife-edge,  whether  of  steel  or  of  agate,  is  made  in 
one  piece,  and  runs  through  a  perforation  in  the  beam  :  if, 
as  is  occasionally  the  case,  the  knife-edge  is  made  in  two 
parts,  one  being  affixed  to  each  side  of  the  beam,  it  becomes 
almost  impossible  to  bring  the  edges  into  exactly  the  same 
straight  line.  4.  Inequalities  in  the  lengths  of  the  arms 
may  be  detected  by  loading  the  pans,  after  the  beam  has 
been  found  to  be  in  equilibrium,  with  counterpoising 
weights,  and  transferring  the  weights  from  one  pan  to  the 
other  ;  if  equilibrium  is  retained,  the  lengths  of  the  arms  are 
equal.  A  final  test  of  the  efficacy  of  the  balance  consists 
in  weighing  an  object  several  times  in  succession  with  the 
greatest  exactitude  which  the  instrument  admits  of:  if  it  is 
trustworthy  the  greatest  differences  will  not  exceed  o'2  mil- 
ligram. 

It  is  very  desirable  that  the  student,  so  soon  as  he  has 
had  a  little  practice  in  weighing,  should  make  himself  tho- 
roughly acquainted  with  the  capabilities  of  the  instrument 
which  he  employs.  An  hour's  careful  observance  of  its 
behaviour  under  varying  conditions  will  materially  conduce 
to  accurate  weighing,  and  save  much  subsequent  time.  He 
should,  in  the  first  place,  determine  for  himself  the  degree 
of  its  sensitiveness  by  accurately  noting  the  deviation  from 
the  zero-point,  which  an  overweight  of  a  milligram  effects ; 
(i)  when  the  balance  is  unloaded,  and  (2)  when  carrying 
50  grams  on  each  pan ;  and  (3)  when  carrying  100 
grams.  He  should  at  the  same  time  observe  the  variation 
in  the  rates  of  oscillation  as  the  load  increases.  As  the 
balance  approaches  equality,  after  some  experience  he 
will  readily  be  able  to  estimate  the  weight  required  to 
establish  equilibrium,  from  the  extent  and  rapidity  of  the  os- 
cillations. 


The  Balance.  1 5 

The  balance-room  should  have  a  northerly  aspect,  and  it 
should  not  be  liable  to  great  or  sudden  variations  in  tem- 
perature. If  possible,  the  instruments  should  be  so  arranged 
within  the  room,  that  in  weighing  the  light  falls  over  the 
right  shoulder  of  the  operator.  The  position  of  the  several 
balances  should  be  fixed,  and  they  should  be  moved  as 
seldom  as  possible,  otherwise  their  adjustment  to  horizon- 
tality  will  be  continually  disturbed,  and  the  shaking  of  the 
beams  and  pans  will  inevitably  interfere  with  the  constancy 
of  their  indications.  Frequent  attempts  at  readjustment 
by  inexperienced  hands  will  certainly  disturb  the  regular 
action  of  the  instrument.  If,  on  commencing  to  weigh,  the 
balance  is  found  not  to  be  in  equilibrium,  the  beam  and 
pans  should  be  lightly  brushed  with  a  camel's-hair  brush, 
and  the  horizontality  of  the  beam  again  tested.  It  should 
be  carefully  borne  in  mind  that  none  of  the  adjustments 
ought  to  be  disturbed  without  sufficient  reason,  and  only 
after  a  proper  inspection  of  the  several  parts  of  the  instru- 
ment. In  a  laboratory  where  the  same  balance  is  used  by 
several  operators,  the  necessary  adjustments  should  be  in- 
variably made  by  the  assistant,  for  no  one  can  have  con- 
fidence in  the  indications  of  an  instrument  which  is  liable  to 
hasty  and  improper  alteration.  A  balance  suffers  more  from 
imperfect  preservation  than  from  proper  usage.  The  fumes 
of  the  laboratory  should  be  carefully  excluded  from  the 
balance-room,  and  the  student  should  never  neglect  to 
securely  close  the  doors  of  the  balance-case  when  he  has 
finished  weighing.  Careful  exclusion  of  acid  fumes  will  do 
much  to  prevent  the  corrosion  of  the  polished  knife-edges 
and  suspensions  :  if  rusting  commences  at  any  one  point  it 
will  rapidly  extend  over  the  entire  surface  of  the  steel.  In 
order  to  diminish  the  humidity  of  the  air  a  small  dish  con- 
taining dried  carbonate  of  potash  or  a  piece  of  freshly-burnt 
lime  should  be  kept  within  the  case.  A  balance  in  constant 
use  will  require  cleaning  about  every  three  or  four  months. 
The  instrument  -should  be  carefully  taken  to  pieces,  and  the 


1 6  Quantitative  Cliemical  A  nalysis. 

loose  parts  dusted  ;  the  beam,  pans,  and  suspensions  should 
be  rubbed  with  a  piece  of  soft  leather,  and  the  movements 
cleaned  and  oiled.  Occasionally  the  agate  planes  will  re- 
quire repolishing  by  the  maker,  as  the  constant  working  of 
the  knife-edges  wears  a  minute  groove  in  them,  easily  per- 
ceptible by  a  lens.  Lastly,  all  delicate  instruments  should 
be  encased  in  a  well-fitting  baize  or  linen  bag  when  not  in 
frequent  use ;  this  will  greatly  tend  to  keep  out  dust  and 
acid  fumes. 

THE   WEIGHTS. 

Since  the  chemist  mainly  concerns  himself  with  the  rela- 
tive weights  of  the  constituents  of  a  substance,  it  is,  for  the 
greater  number  of  his  operations,  a  matter  of  indifference 
what  unit  he  adopts.  He  needs  merely  to  assure  himself 
that  its  multiples  and  submultiples  actually  have  the  values 
which  are  assigned  to  them.  Experience  shows,  however, 
that  it  is  highly  desirable  that  a  common  unit  should  be 
adopted  by  chemists,  and  that  it  should  be  directly  con- 
nected with  some  national  standard.  Accordingly,  we 
employ  almost  exclusively  the  metric  system,  of  which  the 
gram  is  the  standard  weight.  The  reasons  which  have  con- 
duced to  the  adoption  of  this  system  are  (i)  that  the  unit 
is  moderately  small ;  (2)  that  its  multiples  and  submultiples 
are  derived  from  it  by  decimal  multiplication  and  division, 
i.e.  by  the  simplest  possible  system ;  and  (3)  that  it  bears  a 
very  simple  relation  to  the  measures  of  capacity  and  length. 
A  set  of  weights  extending  from  fifty  grams  to  a  milligram 
will  be  found  most  generally  useful.  Such  a  set  should  con- 
tain pieces  of  the  following  denominations  : 

grams  grams  gram  gram  gram 

50  5  0-5  0-05  0-005 

2O                       2  O'2  O'O2  O-OO2 

10                       I  O'l  O'OI  0-001 

IO                       I  O'l  O'OI  O'OOI 

I  O'OOl 

the  entire  twenty-two  pieces  making  up  101  grams.     The 


The 


I  V eights. 


larger  weights,  down  to  one  gram,  are  kept  in  receptacles 
lined  with  velvet  to  prevent  their  being  scratched  \  the  smaller 
ones  are  also  kept  in  separate  compartments  covered  by  a 
plate  of  glass.  The  box  should  be  furnished  with  small 
forceps,  with  which  the  weights  are  invariably  to  be  handled, 
as  they  should  never  be  touched  by  the  fingers  (fig.  7). 

FIG.  7. 


Several  substances  have  been  proposed  as  the  material  for 
weights  ;  for  example,  rock-crystal  and  platinum  have  actually 
been  used  by  reason  of  their  durability.  In  general,  how- 
ever, only  the  smaller  weights  are  constructed  of  platinum, 
the  others  being  made  of  brass  gilded  by  the  electrotype 
process.  If  the  gilding  is  of  moderate  thickness,  and  the 
weights  are  preserved  with  due  care,  they  will  be  found  to 
be  practically  unchanged  even  after  many  years'  use. 
Probably  the  most  convenient  shape  for  the 
brass  weights  is  that  of  a  short  frustum  of 
an  inverted  cone,  to  the  base  of  which  a 
handle  is  attached  (fig.  8).  Beneath  the 
handle  is  a  small  cavity  containing  the 
minute  pieces  of  foil  or  wire  required  to 
adjust  the  weights.  The  smaller  weights  are 
preferably  made  of  pieces  of  stout  platinum  foil,  one  corner 
of  each  being  turned  up  for  holding  in  the  forceps  ;  the  very 


FIG.  8. 


1 8  Quantitative  Chemical  A  nalysis. 

small  ones  are  occasionally  made  of  palladium  or  aluminium. 
All  the  pieces  should  have  their  values  plainly  stamped 
upon  them,  and  the  compartments  of  the  smaller  weights 
should  be  large  enough  to  admit  of  their  being  easily  with- 
drawn ;  otherwise  the  foil  will  soon  become  bruised,  and  the 
denomination  of  the  weight  rendered  indistinct. 

The  greatest  care  should  be  taken  to  preserve  the  weights 
from  the  action  of  acid  fumes.  The  lid  of  the  box  in  which 
they  are  contained  should  be  tightly  fitting,  and  the  box 
itself  should  be  kept  in  a  bag  of  soft  leather.  No  attempt 
should  ever  be  made  to  clean  the  weights  by  rubbing  ;  any 
dust  may  be  removed  by  a  camel's-hair  brush.  If  by  chance 
the  small  platinum  weights  become  dimmed  or  soiled,  they 
may  be  brightened  without  injury  by  holding  them  for  a 
moment  in  the  flame  of  the  Bunsen  lamp.  Brass  weights  un- 
gilded  generally  become  heavier  by  corrosion ;  when  gilded 
they  usually  become  lighter  by  wear.  Since  this  action 
occurs  only  at  the  surface,  which  increases  as  the  square  of 
the  diameter,  whilst  the  mass  increases  as  the  cube,  it 
follows  that  the  smaller  weights  become  more  quickly  erro- 
neous than  the  larger.  The  error  thus  caused  by  age  is, 
however,  exceedingly  minute.  The  slight  tarnish  is  so  ex- 
cessively thin  that  it  requires  a  very  delicate  instrument  to 
detect  its  influence. 

We  strongly  recommend  the  operator  to  test  his  set  of 
weights ;  for  however  carefully  the  remainder  of  his  quanti- 
tative work  may  be  done,  if  he  weighs  with  grossly  in- 
accurate weights,  his  results,  if  not  valueless,  will  at  least  be 
inexact.  The  exact  determination  of  the  values  of  the 
separate  pieces  in  a  set  of  weights  in  terms  of  one  of  them 
selected  as  a  standard  is  an  operation  of  some  skill,  and 
requires  a  considerable  expenditure  of  time.  As  a  rule,  the 
weights  of  the  best  makers  possess  greater  accuracy  than  is 
required  of  them  in  ordinary  quantitative  processes  ;  never- 
theless their  examination  should  never  be  omitted.  The 
readiest  method  of  detecting  errors  in  the  values  of  the 


The  Weights.  19 

denominations  is  to  place  one  of  the  gram  weights  on  the 
pan  of  a  delicate  balance,  adjusted  to  perfect  equilibrium, 
and  equipoise  it  with  pieces  of  brass  or  small  shot,  and 
finally  tinfoil.  The  weight  is  then  removed  and  replaced 
by  the  second  gram  weight,  and  the  balance  caused  tq 
oscillate.  If  the  excursions  on  either  side  of  the  zero  are 
of  equal  amplitude,  the  weights  are  equivalent ;  if  not,  the 
deviation  must  be  noted.  It  should  not  exceed  i  division 
of  the  scale  from  the  zero-point.  The  third  gram  weight  is 
next  tried  in  the  same  way,  and  it  is  then  replaced  by 
platinum  weights  of  the  smaller  denominations  to  make  up 
i  gram,  and  the  balance  again  caused  to  oscillate,  every 
deviation  from  ,the  equilibrium  being  carefully  noted.  In 
the  same  way  the  2  gram  piece  is  compared  with  two  of  the 
single  grams,  the  5  gram  piece  with  the  2  -f  i  +  i  +  i  gram 
pieces,  and  each  of  the  10  gram  pieces  with  the  5  +  2  +  1  +  1  +  1 
gram  pieces.  The  larger  pieces  also  should  not  show  greater 
variations  than  i  division  from  the  zero,  since  the  value  of 
i  scale  division  with  a  heavy  load  on  the  pans  is  almost 
invariably  greater  than  with  a  diminished  load,  for  the  reasons 
already  given.  Thus  in  a  certain  balance  tested  by  the 
author,  i  scale  division,  with  no  load  on  the  pans,  was 
equivalent  to  0-29  milligram  ;  i.e.  a  preponderance  of  0-29 
milligram  would  cause  a  variation  of  i  division  from  the 
zero-point.  Under  varying  loads  the  value  of  i  division 
on  this  balance  was  as  follows  : — 

Load  on  each  pan  Value  of  i  scale  division 

grams  milligram 

10  0-32 

25  0-35 

50  0-39 

100  0-51 

The  process  of  testing  should  be  repeated  from  time  to 
time,  particularly  when  the  weights  begin  to  be  tarnished. 
It  is  also  desirable  that  the  student  should  be  able  to 
compare  his  weights  with  a  standard  set. 

c  2 


2O  Quantitative  Chemical  Analysis. 


THE   OPERATION   OF   WEIGHING. 

There  are  three  methods  of  determining  the  weight  of  a 
body  by  means  of  the  balance :  (i)  by  the  process  of 
direct  weighing,  (2)  by  that  of  reversal,  and  (3)  by  that  of 
substitution.  As  the  first  is  the  most  expeditious,  it  is  the 
one  usually  employed,  although  it  is  not  the  most  accurate 
method.  The  substance  to  be  weighed  is  placed  on  one 
pan,  conveniently  that  on  the  left,  and  the  weights  are 
placed  on  the  other.  The  pan  originally  selected  to  contain 
the  object  to  be  weighed  should  be  used  for  the  same 
purpose  subsequently.  The  effect  of  any  inequality  in  the 
length  of  the  arms  is  thus  in  a  great  measure  obviated. 
Supposing  that  we  wished  to  determine  the  amount  of  copper 
iira  piece  of  the  pure  metal,  and  that  for  the  purpose  of  the 
estimation  we  wished  to  weigh  out  i  gram  of  the  metal,  and 
that  the  arms  of  our  balance  were  of  unequal  length,  the  one 
on  the  left  being  99  millimetres,  whilst  the  other  on  the 
right  was  101  millimetres  long.  We  place  a  i  gram  weight 
on  (say)  the  /(//-hand  pan,  and  counterpoise  it  with  copper. 
Since  two  masses  acting  on  a  lever  are  in  equilibrium  when 
the  products  of  the  weights  into  their  distances  from  the 
fulcrum  are  equal,  it  follows  that  the  weight  of  copper 
equivalent  to  the  i  gram  weight  would  be  0-9802  gram,  for 
99 x  i'o=ioi  x 0*9802.  The  metal  is  next  dissolved  and 
the  copper  precipitated,  these  operations  being  so  carefully 
performed  that  nothing  is  lost.  The  precipitated  copper  is 
then  weighed,  and  by  mischance  it  is  brought  on  the  left-hand 
pan ;  when  counterpoised  with  the  weights,  it  would  appear 
to  weigh  0-9705  gram,  since  99x0-9802  =  101x0-9705: 
accordingly  the  pure  copper  would  appear  to  contain  only 
97 '05  per  cent,  of  metal.  Had  we  invariably  employed  the 
same  scale  for  the  same  purpose,  we  should  have  determined 
what  we  required  to  know — viz.,  that  the  copper  contained 
100  per  cent,  of  the  metal — although  we  might  not  know 
(which  would  be  perfectly  immaterial)  what  was  the  true 


The  Operation  of  Weighing.  21 

weight  of  the  metal  employed  in  the  analysis.  And  by 
similar  reasoning  we  see  that,  in  the  analysis  of  a  substance 
containing  any  number  of  constituents,  we  ought  to  use  one 
and  the  same  pan  as  the  object-pan ;  the  results,  being 
strictly  comparative,  are  thus  independent  of  the  imperfec- 
tion of  the  instrument,  since  the  apparent  weights  of  all  the 
bodies  weighed  are  altered  in  exactly  the  same  ratio. 

We  can,  however,  obtain  the  absolute  weight  of  the  copper 
taken  for  analysis,  either  by  the  method  of  reversal  or  by 
that  of  substitution.  In  the  method  of  reversal  (known  as 
Gauss's)  the  object  (the  copper  for  example)  is  weighed 
first  in  one  pan,  and  then  in  the  other.  If  the  weights  are 
identical,  the  true  weight  of  the  object  is  at  once  given.  If 
the  weights  are  unequal,  their  geometric  mean  will  be  the 
true  weight;  this  is  found  by  multiplying  the  apparent 
weights  together  and  taking  their  square  root.  Practically 
the  common  arithmetic  mean  of  the  two  weights  will  be 
sufficiently  accurate,  unless  their  difference  is  considerable. 

In  the  method  of  substitution  (due  to  Amiot  *),  the  body 
to  be  weighed  is  first  accurately  counterpoised ;  it  is  then 
removed,  and  equilibrium  again  established  by  placing 
weights  in  its  stead.  Obviously  the  absolute  weight  of  the 
body  is  expressed  by  the  value  of  the  weights  substituted. 
In  practice  this  method  of  weighing  may  be  facilitated  by 
using  one  of  the  larger  weights,  heavier  than  the  body  to  be 
weighed,  as  a  counterpoise,  and  adding  weights  to  the  object 
until  equilibrium  is  established.  The  object  is  then  removed, 
and  weights  substituted  until  the  balance  is  again  in  equili- 
brium. The  substituted  weights  express  the  real  weight  of 
the  object. 

Theoretically  these  methods  are  faultless  ;  practically  they 

are  subject  to  at  least  two  minute  errors — one  due  to  the 

impossibility  of  maintaining  the  edges  of  suspension  in  a 

perfectly  uniform  position  on  the  plates  whilst  the  beam  is 

'  being  repeatedly  released  and  arrested,  the   other  due  to 

*  Known  also  as  Borda's, 


22  Quantitative  Chemical  Analysis. 

insensibility.  As  a  balance,  however  sensitive,  requires 
some  weight  to  make  it  turn,  a  difference  equal  to  this  turn- 
ing weight  may  exist  between  weights  apparently  equal. 
The  error  due  to  insensibility  may  be  eliminated  by  weigh- 
ing the  object  several  times  in  succession,  since  the  balance 
ought  to  turn  as  readily  one  way  as  the  other. 

The  weights  should  be  placed  on  the  weight-pan  methodi- 
cally, and  not  taken  haphazard  from  the  box.  A  very  little 
experience  is  sufficient  to  tell,  roughly  speaking,  the  weight 
of  an  object.  Let  us  suppose  that  we  wished  to  determine  the 
weight  of  a  platinum  crucible  which  we  afterwards  found  to 
be  26*715  grams.  We  place  on  the  weight-pan  the  20 
gram  piece  and  release  the  beam — this  proves  too  little ;  we 
add  one  of  the  10  gram  pieces — this  is  too  much  ;  we  sub- 
stitute the  5  gram  piece  for  the  10  gram — the  weight  is  again 
too  little;  we  add  the  2  gram  piece — this  is  again  too  much; 
we  substitute  one  of  the  i  gram  pieces  for  the  2  gram  piece 
— the  weight  is  too  little ;  we  add  the  0*5  gram,  still  too  little; 
also  the  0*2  gram,  still  too  little,  although  we  observe  that 
the  rate  of  the  vibration  of  the  beam  becomes  much  slower 
— the  balance  is  rapidly  approaching  equilibrium.  It  is  at 
this  point  that  the  skill  of  the  operator  comes  into  play  • 
an  expert  weigher,  familiar  with  the  indications  of  his  instru- 
ment, can  almost  intuitively  tell,  from  the  extent  and  rapidity 
of  the  vibrations,  what  weight  is  required  to  establish  equili- 
brium. Until  this  experience  is  acquired  it  will  be  well  to  try 
the  addition  of  the  remaining  weights  in  the  methodical 
manner  above  indicated.  We  find  that  267  grams  is  not  quite 
sufficient  to  equipoise  the  crucible  ;  we  add  0*1  gram — too 
much  (the  pointer  swings  with  increased  energy  in  the 
opposite  direction)  ;  substitute  0-05  for  the  o-i  gram — still 
too  much;  try  cro2 — the  pointer  vibrates  much  more  slowly, 
but  still  indicates  that  the  weight  is  too  great ;  we  substitute 
o'oi  gram  for  the  0*02  gram — the  pointer  swings  with  the 
same  slowness,  but  shows  an  equal  deflection  to  the  opposite 
side  of  the  zero  ;  we  add  '005  gram — the  pointer  now  makes 
excursions  of  equal  amplitude — the  balance  is  in  equilibrium. 


The  Operation  of  Weighing.  23 

Having  assured  ourselves  that  such  is  the  case  by  reading 
off  the  position  of  the  pointer  at  the  end  of  the  vibrations 
several  times  in  succession,  we  finally  arrest  the  motion  of 
the  beam,  and  determine  the  aggregate  value  of  the  weights 
employed,  (i)  from  the  vacant  spaces  in  the  box,  and  (2) 
from  the  denominations  on  the  weights  themselves.  This 
double  reading  should  never  be  neglected  :  the  one  method 
serves  to  control  the  other. 

It  must  be  carefully  borne  in  mind  that  whenever  an 
exchange  of  weight  is  necessary,  the  motion  of  the  beam 
must  be  arrested  :  under  no  circumstances  must  anything  be 
removed  from  the  pans  when  the  balance  is  free  to  oscillate. 
Neglect  of  this  precaution  will  quickly  ruin  the  instrument. 

The  difficulty  of  transferring  and  reading  the  weights  is 
greatly  increased  when  we  arrive  at  such  minute  fractions  as 
the  milligram.  In  order  to  obviate  the  inconvenience  in- 
separable from  the  use  of  a  number  of  small  weights,  which 
are  apt  to  be  lost  or  erroneously  read,  Berzelius  proposed 
to  estimate  the  last  minute  fractions  (the  milligrams  and 
tenths  of  a  milligram)  by  a  small  movable  weight  sliding 
along  the  beam.  A  piece  of  brass  gilt,  aluminium,  or 
palladium  wire,  weighing  exactly  i  centigram,  is  bent  in  the 
form  represented  in  A,  fig.  7  ;  this  weight  is  called  a  rider , 
and  by  means  of  the  movable  rod  (fig.  i )  worked  from  the 
outside  of  the  balance  case  it  can  be  placed  on  one  arm 
of  the  beam  (that  to  which  the  weight-pan  is  suspended)  at 
any  required  distance  from  the  centre  edge.  The  arm  from 
the  centre  to  the  terminal  edge  is  divided  into  ten  equal 
parts,  and  each  of  these  is  (generally)  subdivided  into  five 
equal  parts.  The  rider  weighs  exactly  i  centigram  or  10 
milligrams  when  placed  just  above  the  terminal  knife-edge. 
When  acting  at  a  point  on  the  arm  exactly  midway  between 
the  two  edges,  it  exerts  only  half  this  effect — in  other  words, 
it  now  becomes  equivalent  to  5  milligrams  ;  when  acting  at 
a  fourth  of  the  distance  from  the  centre  edge,  it  is  equal  to 
2-5  milligrams,  at  three-quarters  the  distance  7^5  milligrams, 
and  so  on.  The  employment  of  this  very  simple  contrivance 


24  Quantitative  Chemical  A  nalysis. 

is  attended  with  much  economy  of  time  and  trouble,  and 
with  greater  accuracy,  as  the  rider  allows  the  minutest  varia- 
tion to  be  estimated,  and  diminishes  the  chance  of  error  in 
reading  the  weights. 

The  subdivisions  of  the  centigram  may  also  be  rapidly 
estimated  by  means  of  the  arrangement  represented  in  fig.  9. 

FIG.  9. 


To  the  bottom  end  of  the  rod  carrying  the  screw  c  is  fixed 
a  small  movable  pointer  d,  which  moves  over  a  graduated 
arc.  The  pointer  can  be  pushed  along  the  arc  by  means 
of  the  arm  /  worked  by  the  milled-head  screw  G,  placed  on 
the  outside  of  the  balance  case.  This  pointer  acts  as  a 
small  weight,  its  value  depending  on  its  proximity  to  the 
plane  of  the  beam.  The  balance-maker  effects  the  gradua- 
tion of  the  arc  by  placing  a  centigram  weight  upon  one 
pan,  and  moving  the  index  in  the  direction  of  the  opposite 
pan  until  equilibrium  is  established ;  one  milligram  of  the 
weight  on  the  pan  is  now  removed,  and  the  position  of  the 
index  along  the  arc  again  noted  when  equilibrium  is  re-esta- 
blished. Successive  milligrams  are  removed,  and  the  position 


The  Operation  of  Weighing,  £5 

of  the  pointer  required  to  bring  about  equilibrium  is  repeatedly 
determined.  The  divisions  are  then  subdivided  into  tenths, 
each  representing  0*1  milligram. 

The  following  general  rules  may  prove  serviceable  to  the 
student  in  weighing  :  — 

i.  Before  commencing  to  weigh,  see  that  the  rider  hangs 
upon  the  projecting  pin  of  the  sliding  rod,  and  not  upon  the 


2.  Next  test  the  equilibrium  of  the  balance  by  cautiously 
lowering  the  supports  and  setting  the  beam  in  oscillation. 
If  the  balance  is  not  in  equilibrium,  seek  for  the  cause  of 
disturbance,  and  brush  the  pans,  &c.,  with  a  camel's-hair 
brush,  and  again  test  the  equilibrium  before  attempting  to 
move  the  vane  or  alter  the  terminal  screws. 

3.  When  it  is  necessary  to  arrest  the  motion  of  the  beam, 
or  to  transfer  any  of  the  weights,  or  to  remove  the  body 
weighed,  the  supports  should  be  raised  the  moment  that  the 
pointer  is  opposite  the  zero  of  the  scale.     The  screw  regu- 
lating the  eccentric  should  be  gently  turned  as  the  pointer 
travels  towards  the  zero,  so  that  immediately  they  are  in 
coincidence,  a  very  slight  but  rapid   turn  may  arrest  the 
beam   without  any  jerking   or  vibration.     If  the  screw  is 
suddenly  turned  when  the  pointer  is  at  the  end  of  an  excur- 
sion, the  beam  will  be  jerked,  and  its  original  position  of 
equilibrium,  as  shown  on  the  ivory  scale,  will  inevitably  be 
disturbed.     Carelessness  in  arresting  the  oscillation  of  the 
beam  greatly  interferes  with  the  uniform  behaviour  of  the 
instrument,  and  with  inexperienced  operators  is  the  most 
frequent  cause  of  disarrangement. 

4.  All  shaking  of  the  room  or  table  on  which  the  balance 
stands  should  be  carefully  avoided.    The  operator  should  be 
seated  so  as  to  have  the  ivory  scale  in  direct  line  of  vision, 
but   he  should   also  be  able  to  remove  the  weights,  &c., 
readily,  and  to  work  the  sliding  rod  carrying  the  rider. 

5.  If  the  balance  is  very  nearly  in  equilibrium,  it  occa- 
sionally happens  that  it  refuses  to  vibrate  immediately  after 


26  Quantitative  Chemica 

releasing  the  beam,  and  the  pointer  remains  stationary.  By 
gently  wafting  the  air  down  upon  one  of  the  pans,  vibration 
to  the  required  extent  may  be  readily  set  up.  In  very 
accurate  weighing  it  is  not  always  desirable  to  reopen  the 
doors  of  the  case  for  this  purpose  :  vibration  may  then  be 
brought  about  by  gently  touching  the  beam  or  top  of  the 
rider  with  the  pin  of  the  sliding  rod. 

6.  As  a  general  rule,  the  substance  to  be  weighed  should 
never  be  placed  directly  upon  the  object-pan,  but  should  be 
contained  either  in  a  crucible  or  on  a  watch-glass.     Owing 
to  its  hygroscopic  nature  paper  cannot  be  used  if  the  exact 
weight  of  the  body  is  of  importance  (and  when  it  is  not 
ordinary   scales   should   be    used   instead  of  the  chemical 
balance). 

7.  A  body  should  never  be  weighed  when  warm.     By 
placing  a  warm  object  on  the  pan  the  indications  of  the 
instrument  are  affected  to  a  marked  extent.     The  ascending 
air-current  produced  acts  against  the  object-pan  and  beam 
above  it,  and  the  body  appears  to  weigh  less  than  it  ought 
to  weigh.  The  warm  air,  after  a  time,  also  affects  the  portion 
of  the  beam  against  which  it  strikes,  and  by  increasing  its 
length  disturbs  the  equality  in  the  arms.     All  substances 
condense  upon  their  surface  a  certain  amount  of  air  and 
moisture,  the  weight  of  which  depends  upon  their  tempera- 
ture.    From  this  cause  also  the  weight  of  a  body  when 
warm  is   always   less   than  when  cold.     A  silver  crucible, 
weighing  when  cold  38-880  grams,  was  heated  over  a  lamp, 
and   placed   whilst   hot   on   the  pan ;  it  now  appeared  to 
weigh   only   38*835  grams  :   weighed   again  when   cold,  it 
regained  its  original  weight.     If  the  crucible  had  contained 
0-5  gram  of  a  body  to  be  heated,  a  loss  of  0-045  gram,  or 
9  per  cent.,  would  have  apparently  occurred,  when  in  fact 
the  weight  of  the  body  might  have  been  unchanged. 

A  platinum  crucible  when  rubbed  with  a  dry  cloth  and 
immediately  weighed  always  weighs  sensibly  less  than  after 
half  an  hour's  exposure  to  the  air  of  the  balance  case,  owing 
to  the  condensation  of  the  air  upon  its  surface.  It  is  ad- 
visable, therefore,  to  allow  the  crucible,  if  freshly  wiped,  to 


The  Operation  of  Weighing.  27 

remain  upon  the  balance  pan  or  in  the  case  some  little  time 
before  being  weighed. 

8.  Hygroscopic  substances  must  be  weighed  in  well- 
covered  crucibles,  or  in  stoppered  tubes,  or  between  watch- 
glasses.  Liquids  must  be  weighed  in  covered  vessels  or  in 
stoppered  flasks.  Expedients  required  in  particular  cases 
will  be  mentioned  hereafter. 

Within  certain  limits,  the  deviation  from  the  horizontal 
in  a  balance  is  proportional  to  the  weight  causing  it. 
Advantage  may  be  taken  of  this  fact  to  estimate  the  last 
fractions  of  a  weight  (i.e.  the  parts  of  a  milligram)  with 
great  accuracy.  When  the  balance  is  very  nearly  in  equili- 
brium, it  is  caused  to  oscillate,  and  the  position  of  the 
pointer  when  at  rest  determined  from  successive  observa- 
tions of  the  extreme  points  of  the  vibrations.  So  soon  as 
the  excursions  of  the  pointer  fall  within  a  certain  limit, 
their  extent  commences  to  decrease  at  a  regular  and  uniform 
rate.  Let  #„  a^  a3  .....  be  the  extreme  points  con- 
secutively reached  by  the  pointer  in  its  oscillations  ;  then  the 
equilibrium  of  the  balance  x  is 


r  =  - 


n 


If  we  know  the  weight  corresponding  to  a  given  deviation 
from  the  zero,  the  estimation  of  the  minute  fraction 
required  for  exact  equilibrium  becomes  an  easy  problem. 
We  have  first  to  determine  the  values  of  one  division  of  the 
graduated  scale  (i.e.  the  weight  required  to  make  the  pointer 
deviate  one  division)  for  varying  loads.  The  balance,  having 
been  adjusted  as  nearly  as  possible,  is  made  to  oscillate,  and 
the  extreme  positions  of  the  pointer  in  its  excursions  observed 
through  a  telescope.  An  odd  number  (conveniently  seven) 
of  successive  readings  are  made  so  soon  as  the  pointer  reaches 
division  6  on  the  scale  :  the  arithmetical  mean  of  the  half- 
differences  between  consecutive  pairs  of  observations  gives 
the  position  of  rest  of  the  pointer  along  the  graduated 
scale. 


28  Quantitative  Chemical  A  nalysis. 

In  an  actual  determination  of  the  position  of  rest  (x)  the 
following  readings  were  made.  The  deviations  of  the  pointer 
from  the  zero  are  marked  +  when  they  occur  to  the  left  of 
the  observer,  and  —  when  they  occur  to  his  right : 

+  5'5 

-  4-5  +  0-50 
+  5-0  +  0-25 
—  4-0  +  0-50 
+  4-5  +  0-25 

-  3'5  +  0-50 
+  4fo  +  0-25 

*  =  ^  =  +  °'375 

An  overweight  of  0-5  milligram  is  then  made  to  act  on  the 
beam,  the  balance  again  set  in  oscillation,  and  successive 
readings  again  taken  : 

Example  :  +8-5 

-  2-0  +    3-25 
+  7'5  +    275 

-  1-0+    3-25 
+  67  +    2-85 

-  07  +    3-00 

+  6'2 


6 

The  overweight  is  then  removed,  and  the  position  of 
equilibrium  again  determined  :  the  second  determination 
usually  differs  to  a  slight  extent  from  the  original  one,  owing 
to  unavoidable  variations  in  the  relative  positions  of  the 
plates  and  edges.  The  mean  position  is  therefore  taken  as 
the  true  point.  In  the  case  cited  the  second  determination 
gave  +  0-260  :  accordingly  the  mean  point  is  +  0-3 1 7. 
Then  the  deviation  due  to  the  overweight  of  0-5  milligram 
would  be  2'975  —  0*317  =  2-658  divisions;  or  i  division 
of  the  scale  would  be  equivalent  to  0-188  milligram  (£).  An 
overweight  of  i  milligram  is  next  caused  to  act  on  the  beam, 
and  the  balance  is  again  made  to  vibrate.  This  weight 
ought  to  produce  double  the  amount  of  deviation  caused  by 


Weighing  by  Vibrations.  29 

the  o'5  milligram  :  if  any  difference  is  observed,  the  mean  of 
the  two  observations  is  taken  as  representing  the  true  value 
of  S.  The  determination  of  5  must  be  made  with  varying 
loads,  for,  as  already  explained,  the  sensibility  of  a  balance 
is  seldom  constant.  In  the  instrument  which  gave  the  fore- 
going readings  the  sensibility  increased  with  the  load : 

Load  Value  of  8 

grams  milligrams 

o-          0-209 

10  0'202 

50  0-188 

It  would  obviously  be  absurd  to  employ  such  a  refined 
method  of  weighing  unless  we  are  assured  that  the  differ- 
ences in  the  relative  values  of  the  weights  we  use  fall  within 
the  errors  of  observation.  However  good  a  set  of  weights 
may  be,  the  values  of  the  several  pieces  are  never  in  exact 
accordance  with  their  denomination  :  the  50  gram  piece,  for 
example,  is  seldom  if  ever  exactly  fifty  times  the  weight  of 
each  of  the  i  gram  pieces,  nor  has  each  of  the  10  gram 
pieces  exactly  -1th  of  the  weight  of  the  50  gram  piece.  The 
method  of  determining  minute  weights  by  vibrations,  as 
above  described,  affords  a  simple  means  of  comparing  the 
pieces  in  a  set  of  weights,  and  of  estimating  their  true  values. 
A  delicate  balance,  placed  in  a  room  as  little  subject  as 
possible  to  vibrations  and  changes  of  temperature,  is  care- 
fully adjusted,  and  the  value  of  S  determined  on  it  in  the 
manner  already  described — (i),  with  the  pans  empty;  (2), 
with  a  load  of  10  grams;  (3),  with  one  of  20  grams;  and 
(4),  with  one  of  50  grams.* 

*  In  these  determinations  the  greatest  care  is  necessary  to  preserve 
the  balance  under  perfectly  uniform  conditions.  The  operation  should 
be  conducted  in  a  room  (best  in  a  cellar)  set  apart  for  the  purpose.  If 
the  instrument  is  exposed  to  a  sudden  change  of  temperature,  its  equi- 
librium will  almost  inevitably  be  disturbed,  owing  to  the  unequal  expan- 
sion of  its  arms.  A  rise  of  temperature  also  affects  its  sensibility  (i)  by 
increasing  the  distance  between  the  centre  edge  and  centre  of  gravity 
and  (2)  by  flexure  of  the  beam.  The  value  of  one  scale  division  on  a 
delicate  balance  is  invariably  greater  in  summer  than  in  winter. 


3O  Quantitative  Chemical  Analysis. 

A  slight  mark'  is  made  with  a  sharp-pointed  needle  on  one 
of  the  lo-gram  pieces,  best  near  its  number  :  similarly  one 
of  the  i  gram  pieces  is  marked  ',  the  other  is  marked  ".  One 
of  the  two  platinum  OT  gram  pieces  and  one  of  the  0*01 
gram  pieces  have  each  a  second  corner  turned  up  j  these 
weights  are  respectively  styled  o'i'  and  ofoi'  gram.  The 
object  of  these  markings,  &c.,  is  to  enable  the  weights  to  be 
again  recognised.  One  of  the  weights  (say  the  unmarked 
10  gram  piece)  is  considered  as  normal;  this  is  placed  on 
the  pan  to  the  right  of  the  operator,  and  is  tared  with  a  piece 
of  the  same  denomination  from  a  similar  set.  The  beam  is 
•then  cautiously  released  and  made  to  vibrate,  the  excur- 
sions of  the  pointer  being  observed  through  a  telescope 
placed  at  a  convenient  distance  (10  or  12  feet)  from  the 
balance.  The  position  of  rest  x  is  deduced  from  the  read- 
ings in  the  manner  already  described.  The  10  gram  is  then 
replaced  by  the  10'  gram  piece,  the  balance  is  again  caused 
to  oscillate,  and  a  second  set  of  readings  taken.  The  adopted 
standard  is  again  placed  on  the  pan,  and  a  third  set  of  obser- 
vations made,  and  again  the  10'  gram  piece  is  substituted 
and  another  set  of  readings  taken.  From  the  mean 
position  of  rest  (x')  deduced  from  the  series  with  the  10' 
gram  piece,  we  determine  the  direction  and  extent  of  its 
difference  from  the  adopted  standard,  i.e.,  the  unmarked 
10  gram  piece.  The  following  mean  results  of  actual  read- 
ings will  serve  to  show  the  degree  of  uniformity  which  may 
be  expected : 

Tare  v.  10  gram.  Tare  v.  10'  gram. 

Observation             x  Observation             x! 

I             +  3-09  II             +  3'10 

III          +  3-09  IV          +  3-16 

V          +  3-09  VI          +  3-11 

vii       +  3-09 

Mean  +  3*09  Mean  +  3-12 

It  appears,  therefore,  that  the  lo'gram  weight  is  0*03  of  a 
division  heavier  than  the  normal  weight.  Under  a  load  of 
10  grams  one  scale  division  on  this  balance  corresponded  to 


Weighing  by  Vibratiohs. 


o*32  milligram  ;  accordingly  the  10'  gram  piece  weighs 
10  +  0-00032  x  -03  grams,  or  lo'ooooi  grams  when  the  10 
gram  piece  is  taken  as  normal.  The  set  5  +  2+1  + I'+i" 
gram  pieces  is  in  like  manner  compared  with  the  standard  10 
gram  piece.  The  following  mean  readings  were  actually- 
obtained  : 

Tare  v.  standard  10  gram  Tare  v.  5  +  2  +  1  +  1'  +  ! 


II  +  25 

IV  +  2-82 

VI  +  2-90 

VIII  +  2-82 

Mean  +  2*85 


I  +  3-08 

III  +  3'II 

V  +  3-09 

VII  +  3-10 

Mean  +  3-09 

Hence  it  appears  that  the  series  5  +  2  +  i  +  i/  +  i"  is 
lighter  than  the  standard  by  0-24  of  a  division,  and  ac- 
cordingly weighs  IO-  — (0*00032  x  0*24)  or  9-99992  grams. 
The  5  gram  piece  is  then  repeatedly  compared  with  the 
2  +  1  +  1'  +  i"  series,  and  the  2  gram  piece  with  the  i  + 1' 
and  i  +  i"  and  i'  + 1"  pieces,  and  so  on,  the  higher  and 
lower  denominations  being  compared  in  exactly  the  same 
manner.  The  results  of  the  comparisons  are  thrown  together 
in  a  table  which  should  accompany  the  set  of  weights  :  in 
using  these  the  sum  of  the  corrected  values  of  the  several 
denominations  is  taken  as  the  true  relative  weight  of  the 
object  weighed.  This  table  may  conveniently  resemble  the 
one  annexed,  which  contains  the  results  of  a  comparison  of 
a  remarkably  good  set  of  weights  by  Staudinger  of  Giessen. 
D= Denomination  of  weight. 
W=True  relative  value. 


D 

W 

D 

W 

D 

W 

D 

W 

100 

99-99971 

5 

5-OOOO2 

P*5 

0-50002 

0-02 

0-02002 

.So 

49-99971 

2 

I  -99997 

O'2 

•19997  o-oi 

O'OIOOI 

20 

19-99989 

I 

0-99995 

O'l 

•IOOOI 

O'OI 

O'OIOO2 

IO 

lO'OOOOO 

i' 

0-99998 

o-i' 

-09999  !o-oi*i  0-00996 

10' 

10-00001 

I" 

i  -ooooo 

O-O5 

•05001  ooi*  0-01004 

*  Riders 

32  Quantitative  Chemical  Analysis. 

The  determination  of  the  weight  of  a  body  with  the 
greatest  attainable  accuracy  is  a  problem  of  no  slight 
difficulty.  It  not  only  demands  on  the  part  of  the  operator 
considerable  skill  and  a  thorough  acquaintance  with  his  in- 
strument, but  also  the  knowledge  of  certain  numerical  data, 
some  of  which  indeed  can  only  be  approximately  known  to 
him.  Every  substance  immersed  in  a  fluid  is  apparently 
diminished  in  weight  by  the  weight  of  the  fluid  displaced  \ 
and  since  all  our  weighings  are  made  in  the  fluid  which 
everywhere  surrounds  us,  viz.,  the  air,  a  balance  carrying  two 
weights  in  equilibrium  simply  shows  that  the  weight  of  the 
body  weighed  less  the  weight  of  the  air  which  it  displaces  is 
equal  to  the  aggregate  values  of  the  weights  less  the  total 
volume  of  air  which  they  displace.  Since  every  body  dis- 
places so  much  air  as  is  contained  in  the  space  it  occupies, 
it  follows  that,  in  order  to  determine  the  true  weight  of  an 
object  weighed  in  air,  we  must  know  also  : 

1.  The  volume  of  the  body  weighed  (v). 

2.  The  total  volume  of  the  weights  (v'). 

3.  The  weight  of  a  given  volume  of  dry  air  under  stand- 
ard conditions  (L). 

The  weight  of  the  object— z>L=the  aggregate  values  of 

the  weights  —  2/L, 
or 

the  true  weight  of  the  object=the  aggregate  values  of  the 
weights  +  VL  —  Z/L. 

When  the  volume  of  the  body  weighed  is  equal  to  that  of 
the  weights  employed,  Z^L— -Z/L=O:  in  this  case  only  does 
the  balance  directly  give  the  true  weight  of  a  body.  When 
the  volume  of  the  body  weighed  is  less  than  that  of  the 
weights,  the  expression  (Z^L  — V'L)  is  negative  :  the  apparent 
weight  of  the  object  is  greater  than  its  real  weight.  On  the 
contrary,  when  the  volume  of  the  body  weighed  is  greater 
than  that  of  the  weights,  the  apparent  weight  is  less  than  the 
real  weight,  since  VL  is  greater  than  V'L. 


Displacement  of  A  ir.  3  3 

We  can  determine  v  and  v'  either  from  the  linear  dimen- 
sions of  the  bodies,  or  more  easily,  and  more  accurately,  from 
their  densities.  The  value  of  L  requires  correcting  for  varia- 
tion in  temperature,  pressure,  and  atmospheric  moisture. 
We  have  therefore  to  observe  the  thermometer,  barometer, 
and  hygrometer  at  the  moment  of  weighing  :  v  and  v'  are 
also  not  invariable  but  are  dependent  on  the  temperature  at 
the  time  of  weighing ;  we  require  therefore  (when  the  greatest 
possible  accuracy  is  desired)  to  correct  for  their  expansion. 

It  is  seldom  necessary  to  correct  the  indications  of  the 
balance  to  this  extent,  since  it  is  only  in  the  estimation  of 
the  combining  weights  of  the  elements,  and  in  certain 
physico-chemical  determinations  that  such  extreme  accuracy 
is  needed.  In  such  cases  Table  XI.  in  the  Appendix  will 
be  found  useful.  The  scope  of  this  work  will  not  permit  of  a 
fuller  discussion  of  the  precautions  and  corrections  necessary 
in  the  exact  determination  of  weight.  We  would  refer  for 
more  complete  information  to  a  memoir  by  Bessel,  in  the 
'  Astronomische  Nachrichten,'  vol.  vii.,  or  to  Schumacher's 
paper,  'Ueber  dieBerechnung  der  beiWagungenvorkommen- 
den  Reductionen '  (Hamburgh,  1838),  in  which  ^principles 
of  the  corrections  are  very  fully  explained  :  the  physical  data, 
however,  need  revision.  In  Kuppfer's  work,  *  Travaux  de  la 
Commission  pour  fixer  les  Poids  et  Mesures  de  Russie '  (St. 
Petersburg,  1841),  is  given  a  full  account  of  the  method  of 
vibrations;  and,  lastly,  in  Prof.  W.  H.  Miller's  classical 
memoir,  *  On  the  construction  of  the  New  Imperial  Standard 
Pound'  ('Phil.  Trans.'  Part  III.  for  1856),  the  best  manner 
of  conducting  Gauss's  method  of  reversal  is  described  ;  the 
tables  of  correction  therein  given  are  based  on  the  most 
accurate  data. 

GENERAL  PRELIMINARY  OPERATIONS. 

Before  we  commence  any  quantitative  investigation  it  is 
desirable  that  we  should  have  a  clear  conception  of  its 
object — that  we  should  understand  the  question  our  inquiry 
is  intended  to  settle.  If  we  steadily  bear  in  mind  the 

D 


34  Quantitative  Chemical  A  nalysis. 

reason  of  our  labour  we  shall  be  guided  in  the  proper 
selection  of  the  specimen  of  the  substance  which  we  desire 
to  analyse.  Supposing  that  we  have  a  mineral,  and  wish  to 
determine  its  composition  with  the  object  of  elucidating  its 
constitution,  we  ask  ourselves  in  the  outset — would  an  analysis 
of  this  particular  specimen  afford  a  proper  solution  of  the 
question  ?  We  examine  it  with  a  lens,  or  by  some  other 
appropriate  means,  to  learn  if  it  is  free  from  foreign 
matter ;  if  it  is  imbedded  in  a  matrix,  we  carefully  remove 
the  adhering  gangue ;  we  then  break  up  the  mineral,  and 
select  the  cleanest  and  apparently  purest  pieces ;  in  short, 
we  do  everything  that  the  nature  of  the  case  suggests  to 
assure  ourselves  that  we  have  an  individual  body  to  analyse, 
and  not  a  mixture  of  substances. 

Again  let  us  suppose  that  we  are  called  upon  to  examine  a 
cargo  of  ironstone,  or  other  heterogeneous  mass,  with  the  view 
of  ascertaining  its  value.  We  should  not  content  ourselves 
with  examining  the  first  lump  of  the  mineral  which  came 
under  our  notice,  but  we  should  carefully  select  and  mix  a 
sufficient  quantity  from  various  parts  "of  the  mass,  reduce 
the  mixture  to  coarse  powder,  thoroughly  intermix  it,  and 
then  take  a  portion  for  analysis. 

In  the  plan  of  instruction  given  in  this  book,  the  first 
work  of  the  beginner  in  quantitative  analysis  is  the  determi- 
nation of  the  constituents  of  simple  and  definite  compounds 
of  which  the  composition  is  already  established.  One  of  the 
objects  of  these  exercises  is  to  afford  him  the  means  of 
gauging  his  progress  in  manipulative  skill.  To  this  end  the 
substances  to  be  analysed  must  actually  contain  only  those 
constituents  they  are  represented  to  contain  ;  in  other  words, 
they  must  be  pure.  If  their  purity  cannot  be  guaranteed, 
the  main  object  of  these  exercises  is  missed  :  the  student  is 
not  in  a  position  to  compare  his  experimental  results  with 
the  supposed  composition  of  the  body,  and  from  the  want  of 
a  sure  control  he  fails  to  acquire  that  degree  of  proper  con- 
fidence in  his  work  which  every  operator  ought  to  possess. 


Mechanical  Division.  35 

Many  of  the  operations  of  quantitative  analysis  are  of 
continual  recurrence.  They  may  therefore  be  most  conveni- 
ently described  in  this  introductory  part. 

Mechanical  Division. — In  order  to  render  the  substance 
we  wish  to  analyse  more  susceptible  to  the  action  of  solvents 
or  fluxes,  it  is  generally  desirable  to  reduce  it  to  a  more  or 
less  finely-divided  condition.  This  operation  is  usually  con- 
ducted in  mortars.  The  kind  of  mortar  to  be  employed 
depends  upon  the  hardness  of  the  substance  ;  in  all  cases 
the  material  of  the  mortar  and  pestle  must  be  considerably 
harder  than  the  body  to  be  powdered,  otherwise  the  sub- 
stance to  be  analysed  will  be  inevitably  contaminated  with 
the  material  of  the  mortar.  In  general,  smooth  porcelain 
mortars  suffice  for  pounding  salts,  whilst  most  minerals 
require  to  be  powdered  in  mortars  made  of  agate.  The 
agate  mortar  and  pestle  should  be  free  from  cracks  or 
crevices :  the  pestle  may  be  conveniently  inserted  into  a 
wooden  handle,  which  renders  it  much  easier  to  use.  Very 
hard  substances  should  first  be  broken  into  small  pieces  by 
wrapping  them  in  paper,  and  striking  them  with  a  hammer 
upon  a  smooth  surface  of  iron.  The  pieces  should  then 
be  reduced  to  coarse  powder  in  the  steel  mortar  (fig.  10). 
a  is  a  solid  block  of  steel,  into  the  slight 
cavity  at  the  top  of  which  fits  the  hollow 
cylinder  b ;  in  this  cylinder  are  placed  the 
pieces  of  the  substance  to  be  crushed.  The 
solid  pestle  c  is  then  placed  in  the  cylinder, 
and  repeatedly  struck  with  a  hammer  until 
the  pieces  are  sufficiently  broken  up.  Some 
minerals  which  suffer  no  change  on  ignition 
(e.g.  quartz  containing  gold)  may  be  disinte- 
grated by  being  repeatedly  heated  and 
thrown  into  cold  water. 

In  order  to  obtain  the  complete  decomposition  of  many 
minerals  and  insoluble  bodies  by  the  action  of  fluxes,  it  is 

D  2 


36  Quantitative  Chemical  A  nalysis. 

necessary  to  reduce  them  to  the  finest  possible  state  of  sub- 
division. This  was  formerly  frequently  effected  by  the 
process  of  elutriation.  The  substance  was  triturated  with  a 
little  water  in  an  agate  mortar,  and  the  pasty  mass  thrown 
into  a  quantity  of  distilled  water.  After  settling  for  a  minute 
or  two  the  turbid  liquid  was  poured  into  another  vessel,  and 
the  subsident  portion  was  rinsed  back  into  the  mortar  and 
again  triturated  ;  this  process  being  repeated  until  a  sufficient 
amount  of  the  suspended  substance  was  obtained.  After 
standing  for  a  few  hours,  to  allow  the  finely-divided  matter 
to  subside,  the  supernatant  liquid  was  decanted  off,  and  the 
powder  thoroughly  dried.  This  process  is  now  less  frequently 
employed  in  quantitative  analysis  than  formerly,  for  the 
reason  that  it  is  found  that  very  few  substances  are  entirely 
unacted  upon  by  water;  even  finely-divided  felspar  and 
granite  give  up  a  portion  of  their  constituents,  and 
powdered  glass  loses  weight  considerably  when  thus 
treated  with  water.  In  the  case  of  mixed  substances  it  very 
generally  happens  that  some  portions  are  more  easily 
reduced  to  powder  than  others,  and  that  some  have  a  very 
different  specific  gravity  from  others ;  hence  it  is  always 
doubtful  if  a  complex  substance  after  elutriation  has  exactly 
its  original  composition. 

The  majority  of  bodies  may  be  obtained  sufficiently 
finely  divided  by  patient  pounding  and  careful  sifting.  The 
sifting  is  best  effected  through  fine  cambric  or  muslin.  A 
piece  of  the  clean  and  dry  fabric  is  tied  over  a  beaker,  about 
10  centimetres  in  diameter,  and  the  powder  is  thrown  upon 
the  cover,  which  is  then  gently  tapped  with  a  glass  rod.  That 
which  fails  to  pass  through  the  cover  is  again  triturated  and 
sifted,  the  process  being  repeated  until  the  entire  mass  has 
passed  through  into  the  beaker.  The  powder  is  again  to  be 
returned  to  the  mortar  in  small  portions  at  a  time  (using  not 
more  than  will  cover  a  sixpenny  piece),  and  ground  until 
every  trace  of  grittiness  has  disappeared,  and  the  substance 
cakes  in  an  impalpable  dust  round  the  pestle. 


Desiccation. 


37 


FIG. 


Desiccation. — It  has  already  been  stated  that  before  we 
can  proceed  to  analyse  a  substance  we  must  be  assured  that 
it  is  free  from  all  unessential  constituents.  The  most 
frequent  of  these  unessential  constituents  is  moisture,  by 
which  term  we  also  understand  the  water  over  and  above 
that  which  may  be  proper  to  the  constitution  of  the  com- 
pound. This  mechanically-held  water  may  be  due  to  the 
method  by  which  the  body  has  been  prepared,  as  in  the 
crystallisation  of  salts,  or  it  may  be  derived  from  the  atmo- 
sphere, as  in  the  case  of  certain  minerals.  The  majority  of 
substances  require  to  be  dried  before  they  can  be  analysed 
quantitatively.  The  method  by  which  this  can  be  most  pro- 
perly and  readily  accomplished  depends  upon  the  nature  of 
the  body.  If  the  substance  contains  water  of  crystallisation, 
repeated  pressure  between  folds  of  filter-paper  often  suffices 
to  remove  the  moisture.  Occasionally  it  will  be  better  to  place 
the  finely  pulverised  body  in  an 
artificially  dried  atmosphere,  over 
some  hygroscopic  substance,  such 
as  calcium  chloride  or  strong  sul- 
phuric acid.  Fig.  1 1  represents 
a  convenient  form  of  drying  appa- 
ratus or  desiccator^  as  it  is  often 
termed.  It  consists  of  a  glass 
bell-jar  with  ground  and  greased 
rim,  resting  on  a  plate  of  ground 
glass ;  the  dish  is  partly  filled  with 
strong  sulphuric  acid  ;  the  tripod 
may  be  made  of  glass  rod,  and 
the  circular  plate  to  support  the 
dish  or  crucible  containing  the 
substance  of  thin  wood  or  metal.  It  is  sometimes  desirable 
to  hasten  the  desiccation  by  conducting  it  under  diminished 
pressure ;  the  apparatus  has  therefore  an  arrangement  to 
connect  it  with  the  air-pump  or  other  instrument  for  pro- 
curing a  vacuum. 


FIG.  12. 


3  8  Quantitative  Chemical  A  nalysis. 

Fig.    1 2  represents   a  more  portable  form  of  desiccator ; 
it  is  especially  convenient  for  allowing  hot  crucibles,  &c., 

to  cool  in  a  dry  atmosphere  pre- 
paratory to  weighing  them.  The 
lid  and  lower  portion  are  of  glass, 
ground  together,  their  perfect  con- 
junction being  secured  by  a  slight 
film  of  grease.  A  brass  rim,  fitting 
into  the  aperture  of  the  lower 
vessel,  which  contains  sulphuric 
acid  or  calcium  chloride,  carries 
a  triangle  to  support  the  crucible, 
&c. 

Substances  experiencing  no  alteration  in  the  neighbour- 
hood of  100°  may  be  more  quickly  dried  in  the  steam-bath. 

FIG  13. 


Fig.    13  represents  a  simple  and  convenient  form  of  this 
apparatus  ;    it  consists  of  a  chamber   surrounded   on  five 


Desiccation. 


39 


sides  by  an  outer  case  of  sheet  copper,  in  which  the  water 
is  placed,  and  which  only  communicates  with  the  air  at  a 
and  b.  Water  continually  drops  into  a  to  replenish  that 
lost  by  evaporation,  and  the  steam  makes  its  escape  through 
b.  The  atmosphere  within  the  chamber  may  be  renewed 
through  the  holes  c  c  in  the  door. 

If  the  body  bears  a  higher  temperature  without  change, 
it  may  be  heated  in  the  air-bath.     Fig.  14  represents  a  very 

FIG.  14. 


simple  form  of  this  apparatus ;  it  is  made  of  sheet-copper, 
and  is  heated  by  the  lamp  /,  the  flame  of  which  should  be 
surrounded  by  an  earthenware  cylinder,  indicated  by  the 
dotted  lines  in  the  figure.  The  substance  to  be  dried  is 
placed  on  the  shelf  within  the  chamber,  the  temperature  of 
which  is  given  by  the  thermometer  /.  In  certain  analytical 
operations  it  is  desirable  to  maintain  the  bath  at  a  constant 


4O  Quantitative  Chemical  Analysis. 

temperature  for  a  considerable  time  ;  the  flame  must  there- 
fore be  kept  of  a  constant  size,  and  be  corrected  for  varia- 
tions in  the  pressure  of  the  gas.  It  is  very  convenient  when 
the  bath  itself  can  be  made  to  regulate  its  temperature,  so 
that  if  it  becomes  over-  or  under-heated  it  can  momentarily 
cut  off  or  increase  the  supply  of  gas.  The  apparatus  shown 
in  the  figure  is  fitted  with  one  of  the  many  forms  of  regu- 
lators which  have  been  described.  The  U-shaped  tube 
contains  mercury,  into  which  dips  a  tube  connected  with  the 
gas  supply ;  the  gas  from  this  tube  passes  through  a  narrow 
slit,  thence  up  a  glass  tube,  through  the  side-tube  «r,  with 
which  the  caoutchouc  tube  of  the  lamp  is  connected.  By 
means  of  a  loosely-fitting  screw,  the  gas- supply  tube  can  be 
raised  or  depressed  within  the  mercury,  so  that  the  length 
of  the  slit,  and  therefore  the  amount  of  gas  passing  through 
the  apparatus  for  a  given  pressure,  can  be  varied  by 
surrounding  it  with  more  or  less  mercury.  The  other  end  of 
the  U-tube  is  connected  with  a  reservoir  of  air  a,  placed  in 
the  bath.  If  the  screw  is  so  regulated  as  to  maintain  a 
given  temperature  when  the  reservoir  is  heated,  any  increase 
or  diminution  of  this  temperature  will  be  accompanied  by  a 
proportional  increase  or  diminution  in  the  volume  of  air 
withiri  this  reservoir,  and  a  corresponding  rise  or  fall  in  the 
height  of  the  mercury  surrounding  the  orifice  through  which 
the  gas  issues  to  the  lamp.  By  means  of  this  arrangement  a 
uniform  temperature  within  the  bath  (within  2°  at  150°-! 70°) 
may  be  readily  maintained. 

Weighing  out  the  substance. — Having  obtained  it  in  a  fit 
state  for  analysis,  and  having  fixed  upon  the  scheme  of 
separation  to  be  adopted,  the  student  next  weighs  off  a 
certain  amount  of  the  substance,  and  proceeds  to  treat  it  in 
accordance  with  the  requirements  of  his  plan,  No  exact 
general  rules  can  be  given  as  to  the  amount  which  will  be 
required  for  the  analysis,  since  so  much  depends  upon  the 
nature  of  the  body,  and  the  proportion  of  its  several  con- 
stituents. The  greater  the  amount  taken,  the  more  accurate, 


Weighing-  out  the  Substance.  41 

c&teris  paribus,  should  be  the  analysis,  since  the  unavoidable 
errors  in  precipitating,  washing,  and  weighing  do  not  exercise 
the  same  degree  of  influence  on  the  final  result,  when  the 
quantity  of  the  substance  is  large,  as  when  it  is  small.  On 
the  other  hand,  the  smaller  the  quantity  operated  upon  the 
sooner  will  the  analysis  be  finished,  but  at  the  same  time 
the  demand  upon  the  manipulative  skill  of  the  operator  will 
be  increased.  The  object  for  which  the  analysis  is  required 
can  alone  tell  us  how  far  we  should  sacrifice  accuracy  to 
time.  As  the  student  will  glean  from  the  following  examples, 
no  strictly  uniform  plan  can  be  given  of  the  manner  in 
*vhich  substances  should  be  weighed  off  for  analysis;  in 
general,  however,  the  body,  especially  when  in  the  state  of 
powder,  is  weighed  out  from  tubes.  The  light  tube  con- 
taining the  body,  and  fitted  with  a  good  cork,  is  accurately 
weighed,  the  tube  is  removed  from  the  pan,  the  cork  is  with- 
drawn, and  the  proper  quantity  of  the  substance  cautiously 
shaken  out  into  a  beaker  or.  crucible,  as  the  case  demands  ; 
the  cork  is  now  replaced,  and  the  tube  and  its  contents  are 
again  weighed.  The  difference  between  the  two  weighings 
gives  the  amount  of  the  body  taken  for  ^analysis. 

The  further  treatment  of  the  substance  depends  upon  the 
nature  of  the  constituent  or  constituents  to  be  estimated. 
The  experience  to  be  gained  from  the  examples  which 
follow  will  suggest  the  proper  methods.  It  most  frequently 
happens  that  the  body  is  to  be  brought  into  a  state  of 
complete  or  partial  solution,  and  the  constituents  separated 
either  by  evaporation  or  by  precipitation,  or  by  both  pro- 
cesses. Thus  we  can  determine  the  nitre  in  gunpowder  by 
treating  that  substance  with  water,  whereby  the  salt  is 
dissolved,  separating  the  solution  from  the  undissolved 
portion,  evaporating  it  to  dryness,  and  weighing  the  residue. 
We  can  analyse  common  salt  when  in  solution  by  precipita- 
ting the  chlorine  by  the  addition  of  silver  nitrate,  and 
weighing  the  silver  chloride  produced  :  the  solution  still 
contains  the  sodium  (now  as  sodium  nitrate) ;  this,  after 


42  Quantitative  Chemical  Analysis. 

the  removal  of  the  excess  of  the  silver  by  appropriate 
means,  can  be  obtained  by  evaporation.  Precipitation  can 
only  be  resorted  to  when  the  precipitate  is  practically  inso- 
luble in  the  liquid  in  which  it  is  formed,  and  when,  pos- 
sessing a  constant  composition,  it  admits  of  being  freed 
from  foreign  substances,  and  of  being  accurately  weighed. 

Evaporation. — Liquids  are  generally  concentrated  by  evapo- 
ration in  porcelain  basins  placed  over  a  lamp,  care  being 
taken  that  the  solution  never  enters  into  actual  ebullition,  as 
this  would  occasion  loss  by  portions  being  projected  from 
the  dish.  Unless  the  evaporation  is  conducted  in  a  room  set 
apart  for  the  purpose,  it  will  be  advisable,  indeed  actually 
necessary,  if  many  persons  work  together  in  the  laboratory, 
to  protect  the  liquid  from  dust.  A  piece  of  glass  rod  bent 
before  the  lamp  in  the  form  of  a  triangle,  and  covered  with 
a  sheet  of  filter-paper,  and  supported  on  a  stand  over  the 
dish,  forms  an  efficient  shield  (fig.  17,  p.  45).  In  the  evapo- 
ration of  acid  liquids  it  must  not  be  forgotten  that  the 
fumes  may  dissolve  out  the  inorganic  constituents  of  the 
paper  (iron,  lime,  &c.)  and  the  condensed  vapour  dropping 
back  into  the  dish  may  contaminate  the  liquid  with  those 
substances.  In  such  cases  the  paper  used  must  be  freed 
from  these  soluble  matters  by  treatment  with  acid  in  the 
manner  to  be  hereafter  described. 

Liquids  containing  gas,  which  is  evolved  in  bubbles  on 
the  application  of  heat,  are  very  liable  to  sustain  loss  by 
spirting.  In  such  cases  the  dish  should  be  covered  with  a 
large  watch-glass,  and  the  liquid  should  only  be  gently 
heated  so  long  as  the  evolution  of  gas  continues.  When 
it  has  ceased,  the  projected  portions  maybe  rinsed  from  the 
watch-glass  back  into  the  dish.  The  evaporation  of  such 
liquids  may  also  be  conducted  with  less  chance  of  loss  in 
obliquely-placed  flasks,  which  should  only  be  half  filled  :  the 
portions  spirted  strike  against  the  upper  part  of  the  flask, 
and  are  washed  back  again  into  the  main  body  of  the  liquid 
by  the  condensing  steam.  With  the  flask  so  tilted  the  liquid 


Evaporation. 


43 


may  even  be  gently  boiled,  with  a  very  remote  chance  of 
anything  being  projected. 

Occasionally  a  liquid  has  to  be  evaporated  to  dryness  in 
a  platinum  crucible,  in  order  that  the  residue  may  be 
weighed.  If  the  boiling  point  is  much  higher  than  that  of 
water,  the  evaporation  is  best  conducted  by  heating  the 
crucible,  placed  obliquely,  in  the  manner  seen  in  fig.  15, 
the  heat  being  directed  upon  the  crucible  above  the  level 


FIG.  15. 


FIG.  1 6. 


of  the  liquid,  By  placing  the  lid  in  the  position  indicated 
in  the  figure  the  evaporation  is  materially  accelerated  since 
a  current  of  air  is  thus  caused  to  play  over  the  surface  of  the 
liquid.  A  little  piece  of  wire  gauze  placed  on  the  top  of  the 
lamp,  enables  the  smallest  flame  to  be  produced  without  any 
fear  of  the  gas  igniting  within  the  tube.  This  method  of 
evaporation  from  the  surface  is  especially  serviceable  if  the 
liquid  contains  a  precipitate ;  by  heating  the  crucible  at  the 
bottom  it  is  almost  impossible  in  such  a  case  to  prevent  loss  by 


44  Quantitative  Chemical  A  nalysis. 

succussion  or  bumping.  It  is  also  useful  when  the  heated  solu- 
tion has  a  tendency  to  creep  up  the  sides  of  the  crucible ;  the 
liquid  evaporates  as  it  ascends,  and  meets  the  heated  surface, 
and  the  residue  is  prevented  from  passing  out  over  the  rim. 
Or  the  crucible  may  be  placed  in  a  vertical  position  with  the 
lid  partially  over  the  side,  so  that  a  small  flame  placed  beneath 
the  lid  heats  the  extreme  end  to  dull  redness.  By  conduc- 
tion the  whole  lid  becomes  hot,  and  radiates  sufficient  heat 
to  effect  a  tolerably  rapid  evaporation  of  the  liquid. 

But  as  a  rule  it  is  safer  to  conduct  the  evaporation  of 
liquids  over  the  water-bath.  Fig.  16  represents  one  of  the 
simplest  forms  of  this  apparatus  ;  it  consists  of  a  vessel  of 
sheet-copper,  about  1 5  centimetres  in  outside  diameter,  par- 
tially filled  with  water,  and  set  over  a  lamp ;  the  vessel  to  be 
heated  by  the  steam  is  placed  on  the  top.  The  feath  is  fur- 
nished with  a  number  of  flat  rings  of  various  diameters, 
adapted  to  receive  vessels  of  different  sizes.  In  order  to 
guard  against  the  effect  of  inadvertent  evaporation  of  the  water 
in  the  bath,  the  apparatus,  as  represented  in  the  figure,  has 
a  simple  contrivance  for  turning  off  the  gas  when  the  copper 
basin  becomes  dry.  The  lamp  is  provided  with  a  cock,  the 
lever  of  which  is  prolonged  and  weighted  with  lead  :  it  is  kept 
in  position  by  a  piece  of  thin  thread  passing  over  the  rim  of 
the  basin,  and  attached  to  a  hook  at  the  bottom.  When 
the  basin  becomes  dry,  the  thread  chars,  and  breaks,  and  the 
lever  falls  and  shuts  off  the  gas. 

It  is  far  better,  however,  so  to  arrange  the  bath  that 
the  water  is  continually  replenished.  The  apparatus  seen  in 
fig.  17  is  designed  with  this  object.  The  water  flows  in 
from  the  main  at  a  ;  by  raising  or  depressing  the  glass  tube 
which  slides  watertight  through  a  piece  of  caoutchouc  tube 
slipped  over  the  lower  portion  of  a,  any  required  height  of 
water  may  be  obtained  in  a,  and  accordingly  in  the  bath. 
The  overflow  runs  through  />,  and  may  be  carried  away  by 
a  piece  of  attached  caoutchouc  tube.  This  bath  has  also 
a  number  of  flat  rings,  to  suit  vessels  of  various  sizes. 

Bunsen  has  devised  an  excellent  form  of  water-bath,  which, 


Evaporation. 


45 


when  once  regulated,  necessitates  no  attention  on  the  part 
of  the  operator  in  regard  to  the  water  supply.  It  is  repre- 
sented in  fig.  1 8,  p.  46. 

The  bath  A  is  made  of  sheet  copper,   and   is   partially 
filled  with  water,  which  is  heated  by  the  lamp  a,  the  flame 

FIG.  17. 


of  which  passes  into  the  chimney  shown  in  the  figure  1by 
dotted  lines.  The  fresh  water  enters  from  the  apparatus  B. 
This  consists  of  a  wide  glass  cylinder,  nearly  filled  with 
water,  in  which  is  a  float,  the  l6wer  cylindrical  part  of 
which  contains  mercury,  whereby  it  is  maintained  in  a  ver- 
tical position,  and  at  a  certain  height  in  the  water.  Through 
the  upper  opening  dips  a  tube/,  connected  with  the  water 
supply ;  this  tube  is  fastened  to  the  cylinder,  but  the  depth 


46  Quantitative  Chemical  Analysis. 

to  which  it  dips  into  the  float  can  be  so  regulated  that  at 
the  proper  level  of  water  the  float  rises,  and  the  mercury 
cuts  off  the  entrance  of  the  water.  The  water-bath  is  con- 
nected with  the  cylinder  by  means  of  the  tube  e ;  as  the 
water  evaporates,  its  level  in  B  sinks,  whereby  the  float  falls, 
until  the  end  of  the  tube /is  uncovered  by  the  mercury ; 
water  now  enters  and  flows  over  into  B,  and  the  float,  again 

FIG.  18. 


rising,  shuts  off  the  water.  This  apparatus  is  more  espe- 
cially adapted  to  a  large  laboratory,  since  any  number  of 
the  water-baths  may  be  connected  together,  one  cylinder 
and  float  serving  to  replenish  them  all,  without  waste  of  water. 
It  must  not  be  forgotten  that  the  material  of  the  vessel  in 
which  the  evaporation  is  conducted,  is,  in  general,  more  or 
less  attacked  by  the  liquid  ;  it  has  already  been  stated  that 


Evaporation.  47 

even  pure  water  dissolves  very  appreciable  quantities  of 
glass.  The  influence  of  the  matter  dissolved  from  the 
flasks,  &c.,  used  in  the  operations,  is  too  frequently  lost  sight 
of  in  quantitative  analysis  :  there  is  no  doubt  that  the  results 
are  affected  to  a  greater  degree  than  is  usually  supposed. 
Experiment  has  shown,  that,  in  the  case  of  new  vessels,  the 
amount  of  glass  dissolved  by  a  heated  liquid  is  always  greatest 
in  the  first  hour,  and  gradually  diminishes,  until  it  reaches  a 
certain  amount,  after  which  the  quantity  passing  into  solu- 
tion is,  within  certain  limits,  proportional  to  the  time  oY 
action.  The  amount  dissolved  is  proportional  to  the  surface 
on  which  the  liquid  acts,  and  is  independent  of  the  amount  of 
liquid  vaporised,  so  long  as  it  is  maintained  at  the  boiling 
temperature ;  that  is,  the  mere  rapidity  of  the  evaporation  is 
without  influence.  The  amount  dissolved  is  in  proportion 
to  the  temperature  of  the  liquid.  400  c.c.  of  boiling  water 
in  a  glass  flask  of  600  or  700  c.c.  capacity*  dissolved  in 
the  first  hour  8^9  milligrams  ;  in  three  hours,  14*8  milli- 
grams ;  in  six  hours  22-5  milligrams  ;  in  twelve  hours,  32*5 
milligrams ;  in  twenty-four  hours,  53-3  milligrams,  and 
in  thirty  hours,  66*5  milligrams.  The  same  quantity  of 
dilute  hydrochloric  acid  (n  per  cent),  boiling  in  a  similar 
flask,  dissolved  only  4*2  milligrams  in  the  first  hour ;  5*1  milli- 
grams in  the  third  ;  7-3  milligrams  in  the  sixth ;  9-4  milligrams 
in  the  twelfth  ;  and  1 7  'o  milligrams  in  thirty  hours.  Dilute 
hydrochloric  acid  exerts  much  less  action  therefore  than  pure 
water.  Nitric  acid  in  like  manner  exerts  comparatively  little 
action  on  glass.  400  c.c.  of  dilute  ammonia  (9  per  cent), 
dissolved  from  a  precisely  similar  flask  67  milligrams  in 
one  hour ;  in  three  hours,  15*5  milligrams;  in  six  hours, 
25*3  milligrams  ;  in  twelve  hours,  43*9  milligrams  ;  in 
twenty-four  hours,  84-8  milligrams,  and  in  thirty  hours,  99-6 
milligrams.  It  appears  that  the  extent  of  action  of  am- 
monia-water varies  very  slightly  with  its  strength,  A 
solution  of  ammonium  chloride  (7  per  cent.),  dissolved  in  one 

*  Composition  of  glass,  SiOa  73-8,  GaO  8-6,  Na2O  14-0,  K2O  0-60. 


48  Quantitative  Chemical  Analysis. 

hour  4-2  milligrams  ;  in  six  hours,  7-3  milligrams  ;  in  fifteen 
hours,  9 '6  milligrams;  and  in  thirty  hours,  14-6  milligrams.  As 
a  general  rule,  liquids  possessing  an  acid  reaction,  even  when 
they  contain  salts  in  solution,  dissolve  less  of  the  glass  than 
when  they  have  an  alkaline  reaction.  The  comparatively 
small  quantity  dissolved  by  the  ammonium  chloride  solution 
is  in  a  great  measure  due  to  the  fact  that  this  liquid  acquires 
an  acid  reaction  on  boiling,  owing  to  the  dissociation  of  the  sal- 
ammoniac  :  the  liberated  hydrochloric  acid  appears  to  exert 
a  preservative  action  on  the  glass.  Dilute  sulphuric  acid, 
however,  exerts  a  marked  action,  twice  as  strong  indeed  as 
that  of  water.  The  amount  dissolved  by  alkaline  fluids  is 
very  considerable,  even  when  the  quantity  of  alkali  is  small ; 
in  six  hours  400  c.c.  of  a  boiling  liquid,  containing  i  per 
cent,  sodium  carbonate,  dissolved  34-8  milligrams ;  the 
addition  of  -4V  of  a  per  cent,  of  caustic  potash  to  water 
increases  its  action  threefold.  Certain  salts,  as  ammonium 
carbonate,  calcium  chloride,  common  salt,  potassium  chlo- 
ride, nitre,  act  upon  glass  to  the  same  extent  as  water ; 
sulphate  and  phosphate  of  sodium  act  much  more  energeti- 
cally, the  action  of  the  latter  salt  being  six  times  that  of 
water.  Direct  experiment  has  shown  that  the  glass  in  all 
these  cases  is  virtually  dissolved ;  the  liquids  do  not  extract 
any  one  constituent  in  preference  to  others.  Very  little 
difference  is  observed  in  the  action  of  the  liquids  on  glass 
of  varying  composition,  but  the  true  Bohemian  glass,  rich  in 
silica,  and  poor  in  soda,  is  of  all  the  least  attacked.  Porcelain 
vessels  are  scarcely  acted  upon  by  any  heated  liquids,  with 
the  exception  of  the  alkalies,  and  even  in  their  case  the 
action  is  very  much  smaller  than  with  glass  :  therefore 
vessels  of  the  former  material  should  invariably  be  used  in 
evaporations,  &c.,  whenever  circumstances  permit.* 

The  precipitation  of  substances  intended  to  be  collected 
and  weighed  is  usually  effected  in  beakers,  on  account  of 

*  Emmerling,  Ann.  der  Chemi£  und  Pharm,,  i$o.-2$J. 


Precipitation. 


49 


FIG.  19. 


the  facility  with  which  the  bodies  may  be  transferred,  either 
to  the  filter  or  to  the  vessels  in  which  they  are  to  be 
weighed.  In  cases  where  the  liquid  is  strongly  alkaline,  or 
where  it  has  to  be  heated  for  some  time,  it  is  better  to  use 
porcelain  basins.  The  separation  of  the  precipitate  from  the 
liquid  in  which  it  is  formed,  is  accomplished  either  by  decan- 
tation  or  \>y  filtration,  or  by  a  combination  of  these  processes. 
In  general,  where  the  liquid  has  to  be  filtered,  it  is  advisable 
to  allow  it  to  stand  at  rest  for  some  hours  after  the  addition 
of  the  precipitant,  for  the  reasons  :  (i)  that  the  complete 
separation  of  the  substance  in  the  insoluble  form  occurs  only 
after  some  time ;  and  (2)  that  in  certain  cases,  if  thrown  on 
the  filter  immediately  after  precipitation,  it  is  apt  to  pass 
through  the  pores  of  the  paper.  Before  proceeding  to 
separate  the  liquid,  the  operator  must  invariably  assure 
himself  that  the  precipitant  is  in  excess,  by  adding  a  few 
drops  of  its  solution,  and  noting 
if  any  further  turbidity  is  pro- 
duced. The  clarification  of  the 
liquid  on  standing  allows  this 
to  be  ascertained  with  greater 
certainty.  In  cases  where  the 
precipitate  forms  only  after  some 
time,  the  precipitant  must  be 
added  to  a  portion  of  the  su- 
pernatant liquid,  poured  into 
another  vessel. 

The  separation  of  precipitates 
by  decantation  is  but  seldom 
resorted  to,  on  account  of  the 
length  of  time  which  it  occupies, 
and  the  comparatively  large 
amount  of  water  needed  for 
thorough  washing.  If,  however, 

the  precipitate  has  a  high  specific  gravity,  and  is  practically 
insoluble  in  water,  as  in  the  case  of  silver  chloride,  metallic 

E 


50  Quantitative  Chemical  Analysis t 

mercury,  &c.,  the  process  may  be  advantageously  employed. 
The  subsidence  of  the  precipitate  takes  place  most  readily  in  a 
vessel  of  the  form  seen  in  fig.  1 9,  p.  49  :  this  should  be  made 
of  glass  sufficiently  thin  to  be  heated  without  risk  of  fracture, 
since  warming  greatly  accelerates  the  subsidence.  The  clear 
liquid  is  conveniently  removed  by  a  syphon,  the  longer  limb  of 
which  can  be  closed  by  a  pinch-cock,  so  that  when  the  flask  is 
replenished  with  the  washing  fluid,  the  syphon,  being  always 
filled  with  liquid,  can  again  be  set  in  action  without  the 
operator  being  under  the  necessity  of  refilling  it.  The  precipi- 
tate is  then  transferred  by  the  aid  of  the  wash-bottle  to  the 
crucible  or  dish  in  which  it  is  to  be  weighed,  the  fluid  used 
in  the  transference  being  poured  away,  as  far  as  practicable, 
and  the  precipitate  dried.  The  decanted  liquid  should  in- 
variably be  set  aside,  in  order  to  allow  any  of  the  insoluble 
substance  which  may  have  inadvertently  been  carried  over,  to 
subside  :  if  any  is  detected,  it  is  separated  from  the  liquid  in 
the  manner  described,  and  either  added  to  the  main  quantity 
or  weighed  by  itself. 

In  the  majority  of  cases,  decantation  and  filtration  are 
combined ;  that  is,  the  liquid  is  poured  through  the  filter 
without  disturbing  the  precipitate,  and  the  precipitate,  after 
having  been  agitated  with  fresh  washing  water,  is  allowed  to 
subside  during  the  time  occupied  by  the  contents  of  the 
filter  in  passing  through  the  paper.  Filter  paper  should 
permit  of  rapid  filtration,  and  yet  possess  pores  sufficiently 
minute  to  prevent  the  passage  even  of  the  most  finely 
divided  precipitates  ;  it  should,  moreover,  be  as  free  as 
possible  from  inorganic  matter.  The  Swedish  filter-paper, 
with  the  water-mark  '  J.  H.  Munktell/  is  generally  con- 
sidered to  fulfil  these  conditions  in  the  highest  degree.  It 
contains  about  0-4  to  0-6  per  cent,  of  ash,  consisting  of 

Silica         Alumina          Iron  Lime         Magnesia 

•35-16  3-84  45-06  14-09  1-01=99-16.* 

*  From  an  analysis  communicated  by  Mr.  Walter  Dearden,  Owens 
College. 


Filtration. 


FIG.  20. 


The  amount  and  nature  of  the  ash   vary,   however,   with 
different  '  makes  '  of  the  paper. 

The  paper  should  be  cut  into  niters  of  various  sizes  by  the 
aid  of  circular  patterns  made  of  tin-plate  or  sheet- zinc ; 
these  may  with  advantage  have  the  radii  3,  4,  5,  6,  and  8 
centimetres.  The  filters  possessing  these  radii  are  respectively 
designated  as  Nos.  3,  4,  5,  6,  and  8.  The  filters  should  be 
treated  with  dilute  hydrochloric  acid  (which  extracts  nearly 
the  whole  of  the  inorganic  matter,  with  the  exception  of 
the  silica),  and  afterwards  be  thoroughly 
washed  with  water  and  dried.  This 
treatment  with  acid  may  be  conveni- 
ently made  in  the  apparatus  represented 
in  fig.  20.  The  ready-cut  filters  are 
placed  in  the  vessel,  and  covered  with 
dilute  hydrochloric  acid  (i  part  acid  to 
20  of  water),  which  is  allowed  to  act 
for  a  few  hours.  On  opening  the  pinch - 
cock  at  the  bottom,  the  acid  liquid  flows 
away  •  the  filters  are  then  to  be  repeat- 
edly washed  with  water  until  every  trace 
of  acid  has  disappeared,  after  which  they 
may  be  dried  in  the  water-bath. 

The  operator  must  now  determine  the  amount  of  ash  left 
on  burning  the  prepared  filters,  since  this  requires  to  be  sub- 
tracted from  the  final  weighing  of  the  separated  substance. 
A  light  porcelain  crucible  is  heated  over  the  lamp,  placed  in 
the  desiccator  and  weighed  when  perfectly  cold.  The  cru- 


FlG.  21. 


FIG.  22. 


cible  is  placed  on  a  smooth  glazed  sheet  of  paper,  fig.  21; 
one  of  the  filters  (No.  5,  for  example)  is  repeatedly  folded 


E  2 


52  Quantitative  Chemical  Analysis. 

over  in  plaits  ,of  about  i  centimetre  in  breadth,  and  tightly 
rolled  between  the  finger  and  thumb  from  one  end  of  the 
folded  length  to  the  other  until  it  has  the  form  seen  in  fig. 
22.  About  half  the  length  of  a  piece  of  platinum  wire  40 
centimetres  long  is  wrapped  round  the  rolled-up  filter,  which 
is  now  lighted  at  the  lamp  and  held  over  the  crucible.  The 
flame  quickly  disappears,  and  the  paper  becomes  reduced 
to  a  mass  of  glowing  carbon.  As  soon  as  the  last  spark  has 
died  out — but  not  till  then — the  flame  is  made  to  play  on  the 
ash  held  over  the  crucible,  to  complete  the  combustion  of 
the  carbon.  The  ash  should  now  be  white  or  have  at  most 
a  reddish-gray  tinge,  without  the  least  trace  of  blackness. 
Care  should  be  taken  not  to  heat  the  ash  more  strongly  than 
is  necessary  to  burn  the  carbon,  or  it  may  fuse  to  the  plati- 
num wire.  This  more  readily  happens  with  filters  which 
have  not  been  treated  with  acid,  and  which,  therefore  contain 
comparatively  large  quantities  of  lime,  iron,  &c.  The  ash 
is  now  shaken  out  of  its  platinum  cage  into  the  crucible,  and 
by  tapping  the  wire  against  the  rim  of  the  crucible,  any 
adhering  traces  are  readily  detached.  This  process  is  to  be 
repeated  with  five  additional  filters ;  the  crucible  is  again 
placed  in  the  desiccator,  and  when  cold  re-weighed.  The 
difference  between  the  two  weighings  divided  by  6,  gives  the 
amount  of  ash  left  by  a  No.  5  filter.  Call  this  amount  a :  it 
is  easy  from  this  to  calculate  the  ash  left  by  each  size  of 
filter.  A  No.  5  filter  has  a  radius  of  5  centimetres  ;  its  super- 
ficial area  is  =r27T  or  5x5x3-14=78-5  sq.  centimetres. 
Required,  for  example,  the  weight  of  ash  of  No.  8  filter  (or): 
8  x  8  x  3-14  area=2oo'9  centimetres  ;  and  78-5  :  200*9  *  • 
a  \  x.  It  is  convenient  to  prepare  a  large  number  of  such 
filters  at  a  time,  and  to  calculate  and  arrange  in  a  little  table 
for  use  the  amount  of  ash  left  by  the  different  sizes. 

Glass  funnels  should  always  be  employed  in  quantitative 
analysis:  the  sides  should  be  inclined  at  an  angle  of  60°,  and 
should  be  free  from  irregularities  or  bulgings;  the  stem 
should  not  be  too  short  or  too  wide,  and  the  end  should  be 


Filtration.  53 

cut  obliquely.  The  size  of  the  funnel  to  be  employed  of 
course  depends  upon  the  size  of  the  filter  required ;  the  filter 
must  never  project  beyond  the  funnel ;  it  should  be  within 
one  or  two  centimetres  from  the  edge.  The  size  of  the 
filter  in  its  turn  depends  upon  the  bulk  of  the  precipitate  to 
be  filtered  :  the  precipitate  should  not  occupy  more  than 
half  the  capacity  of  the  filter,  or  the  process  of  washing  will 
be  very  tedious. 

The  filter  paper  should  be  carefully  folded,  and  properly 
placed  in  the  funnel,  moistened  with  hot  water  (unless  cir- 
cumstances forbid  this),  and  pressed  with  the  finger  so  as  to 
cause  it  to  fit  closely  to  the  funnel ;  for  the  better  it  fits,  the 
more  rapidly  will  it  filter,  and  the  less  will  be  the  danger  of 
rupture  on  washing.  The  funnel  is  placed  in  a  convenient 
stand,  so  that  the  edge  of  the  stem  touches  the  side  of  the 
vessel  intended  to  receive  the  filtrate.  By  allowing  the 
liquid  to  flow  down  the  side,  all  splashing  and  consequent 
risk  of  loss  of  the  filtrate  is  avoided.  The  liquid  to  be 
filtered  should  never  be  poured  directly  into  the  funnel,  but 
down  a  thin  glass  rod,  the  stream  being  so  directed  as  to  fall 
against  the  side  of  the  filter  ;  if  poured  into  the  apex,  loss  by 
splashing  will  inevitably  ensue.  The  rim  of  the  vessel  con- 
taining the  liquid  to  be  filtered  should  be  slightly  greased 
with  lard  (free  from  salt) ;  this  prevents  the  chance  of  the 
liquid  running  down  the  outside  of  the  vessel.  Whilst  not 
in  use,  the  rod  is  placed  in  the  vessel  containing  the  preci- 
pitate, or,  if  this  ought  not  to  be  disturbed,  in  a  little  flask  or 
beaker,  which  is  afterwards  rinsed  out  into  the  filter  so  soon 
as  the  whole  of  the  precipitate  has  been  transferred.  It  is 
advisable  to  cover  the  various  vessels  with  glass  plates 
during  the  progress  of  the  filtration,  to  prevent  dust  falling 
into  them.  The  plate  covering  the  beaker  in  which  the 
filtrate  is  received  must  of  course  have  a  small  hole  at  the 
side  to  admit  the  stem  of  the  funnel ;  this  may  be  readily 
snipped  out  by  a  pair  of  pliers,  or  by  a  key,  the  wards  of 
which  allow  of  the  insertion  of  the  glass. 

It  frequently  happens  that  small  particles  of  the  precipitate 


54 


Quantitative  Chemical  Analysis. 


firmly  attach  themselves  to  the  sides  of  the  vessel  and  can- 
not be  rinsed  out  on  to  the  filter.  To  remove  them,  the  end 
of  the  glass  rod  should  be  covered  with  a  short  piece  of  thin 
unvulcanised  caoutchouc  tubing  (about  i  centimetre  long):  by 
rubbing  this  against  the  sides  of  the  vessel,  the  last  traces  of  the 
precipitate  may,  generally,  be  readily  detached.  Or,  instead 
of  the  rod  coated  with  india-rubber,  a  feather  may  be  used,  the 
plumules  of  which  have  been  torn  away  to  within  2  centi- 
metres of  the  end  ;  those  remaining  are  to  be  cut  parallel  to 
the  quill  and  within  -5  centimetre  of  it.  In  transferring 
precipitates  from  a  basin,  the  little  finger  may  be  often  ad- 
vantageously used  to  rub  away  any  of  the  substance  from  the 
sides.  In  all  cases  it  must  not  be  forgotten  that  the  rubbing 
instrument,  after  use,  must  in  its  turn  be  carefully  rinsed. 
If  the  substance  cannot  be  detached  by  mechanical  means, 
it  must  be  re-dissolved  and  again  precipitated. 

The  form  of  wash-bottle  best  adapted  for  use  in  quantita- 
tive analysis  is  seen  in  fig.  23  ;  as  the  nozzle  is  moveable, 


FIG.  23. 


FIG.  24. 


the  jet  may  be  directed  to  any  required  spot.  Fig.  24  shows 
another  kind  of  wash-bottle  with  moveable  nozzle  :  it  is 
especially  convenient  for  washing  down  the  precipitate  from 
an  inverted  beaker  held  over  the  funnel.  The  orifice  of  the 
nozzle  should  not  be  too  large,  or  the  amount  of  water  re- 


Filtration.  5  5 

quired  to  bring  a  precipitate  on  to  the  filter  becomes 
unnecessarily  great.  In  washing  a  precipitate  on  the  filter, 
the  stream  should  be  directed  round  the  edge  of  the  paper, 
and  care  should  be  taken  that  the  force  of  the  jet  is  not  so 
great  as  to  rupture  the  paper.  Carelessness  in  directing  the 
jet  will  inevitably  cause  portions  of  the  precipitate  to  be  pro- 
jected out  of  the  funnel.  The  operator  should  also  guard 
against  the  formation  of  channels  in  the  mass  of  the  precipi- 
tate, through  which  the  water  tends  to  flow  without  coming 
into  contact  with  the  bulk  of  the  substance.  He  should  never 
refill  the  filter  with  liquid  until  the  previous  quantity  has 
passed  through  :  neglect  of  this  rule  not  only  retards  the 
process  of  washing  ;  but  often  occasions  a  turbid  filtrate.  As 
a  rule  hot  water  should  be  employed  in  washing ;  its  use 
accelerates  the  process  greatly ;  the  few  cases  in  which  it  is 
objectionable  will  be  mentioned  hereafter.  In  order  that  the 
heated  wash-bottle  may  be  conveniently  handled,  a  coil  of 
thick  string,  or  some  other  badly  conducting  material,  may 
be  wrapped  round  its  neck. 

Occasionally  the  washing  is  conducted  with  other  liquids 
than  water.  Thus  in  the  estimation  of  potash  and  ammonia 
the  double  platinum  salts  are  washed  with  alcohol,  and  in 
the  determination  of  magnesia  and  phosphoric  acid  the  pre- 
cipitate is  washed  with  dilute  ammonia  water.  One  separate 
wash-bottle  should  be  employed  for  all  the  special  liquids  j 
as  it  will  be  comparatively  seldom  used,  it  may  have  only 
half  the  ordinary  capacity.  By  attaching  a  small  piece  of 
caoutchouc  tubing  to  the  end  of  the  shorter  tube  of  this 
wash-bottle  beneath  the  cork,  cutting  a  slit  through  the 
caoutchouc  to  within  a  centimetre  from  the  end,  and  stopping 
it  nearly  up  to  the  slit  with  a  small  piece  of  glass  rod,  a  simple 
valve  is  formed  which  effectually  prevents  the  escape  of 
the  vapour  of  these  special  washing  fluids — some  of  which, 
like  sulphuretted  hydrogen  and  ammonia,  are  very  irri- 
tating. The  valve  only  opens  by  inward  pressure  and  closes 
immediately  when  this  pressure  is  withdrawn. 

We  cannot  too  strongly  impress  upon  the  beginner  the 


56  Quantitative  Chemical  Analysis. 

necessity  of  conscientiously  performing  the  operation  of 
washing ;  imperfect  or  careless  washing  is  a  very  frequent 
source  of  error  in  quantitative  analysis.  He  should 
invariably  ascertain,  before  he  discontinues  the  opera- 
tion, that  the  liquid  passing  through  no  longer  contains 
any  of  those  substances  which  it  is  the  object  of  the  washing 
to  remove  :  thus  in  the  determination  of  chlorine  as  silver 
chloride,  he  should  test  the  filtered  wash-water  by  adding  to 
a  portion  of  it,  collected  apart  in  a  test-tube,  a  drop  of  dilute 
hydrochloric  acid  ;  if  the  silver  chloride  has  been  washed 
free  from  the  excess  of  silver  nitrate,  not  the  faintest  turbidity 
will  be  produced. 

In  certain  cases,  however,  such  methods  of  testing  the 
perfection  of  the  washing  are  inapplicable.  It  is  obvious 
that  if  we  wash  twice  with  a  given  quantity  of  water, 
we  reduce  the  impurity  more  than  if  we  wash  once  with 
double  the  quantity.  For,  let  the  original  impurity  be 
i  gram,  and  let  us  add  10  grams  of  wash-water  and  filter  off 
10  grams  :  there  will  then  remain  T'Tth  of  the  original 
impurity.  At  the  second  washing  there  will  remain  yTth  of 
that,  or  T^T  of  the  original.  If  we  had  added  the  20 
grams  at  once,  the  impurity  would  have  been  only  reduced 
to  ^T.  It  is  evident  that  for  the  same  amount  of  wash- 
water  we  shall  get  the  best  result  by  using  small  quantities 
at  a  time,  and  washing  many  times. 

The  following  table  gives  an  approximation  to  the  smallest 
volume  of  wash-water,  and  minimum  number  of  washings 
required,  to  reduce  the  precipitate  to  a  given  state  of  purity. 
It  is  obtained  by  regarding  the  apparent  volume  of  the  pre- 
cipitate at  the  bottom  of  the  beaker  or  on  the  filter  as 
consisting  wholly  of  a  solution  of  impure  matter,  which  it  is 
required  to  reduce  to  a  certain  degree  of  purity,  by  successive 
dilutions  with  a  constant  volume  of  water. 

Let  v  be  the  volume  of  the  precipitate  at  the  bottom  of 
the  beaker  or  on  the  filter,  regarded  as  above,  v  the  amount 
of  wash-water  used  at  each  washing,  n  the  number  of 


Filtration. 


57 


washings.    Also  let  *  be  the  fraction  of  the  original  amount 
of  impurity  which  remains  after  n  washings,  then 


Further,  if  w  be  the  total  volume  of  water  employed  in  the 
11  washings,  w=;z  v,  and  (i)  becomes 


If  we  make  n  infinite,  a  well  known  algebraical  theorem  gives 
W  =  v  log€  a       ......     (3) 

and  this  value  of  W  is  the  smallest  volume  of  water  "by  the 

use  of  which  the  impurity  can  be  reduced  to  -  of  its  original 
amount. 


! 

i 

i 

i 

100,000 

50,000 

*  0,000 

10,000 

I. 

II. 

III. 

I. 

II. 

III. 

I. 

II. 

III. 

I. 

II. 

III. 

v 

v 

v 

v 

n 

w 

n 

w 

n 

W 

n 

w 

v 

v 

0-5    2S-4 

14-2 

0-5 

26-7 

i3'3 

o*5 

24-4 

I2'2 

o-s 

227 

if  4 

i     16-6 

16-6 

I5-6 

15-6 

143 

143 

i 

i.V3 

I3-3 

2       ID'S 

2  I'D 

2 

9-« 

197 

2 

9-0 

18-0 

2 

8-4 

16-8 

3 

8-3 

24-9 

3 

7-8 

23-4 

3 

7-1 

21-4 

3 

6-6 

19-9 

4       7'i 

28-6 

4 

67 

26-9 

4 

6-1 

24-6 

4 

57 

22-9 

5  1    6-4 

32-1 

5 

6-0 

30-2 

5'8 

27-6 

ST." 

257 

6 

5  '9 

35  '5 

6 

5-6 

33'4 

6 

S'i 

30'S 

6 

47 

28-4 

7 

svs 

38-8 

7 

S'2 

36-4 

7 

4-8 

33'3 

7 

4'4 

31-0 

8 

5-2 

42-0 

8 

4'9 

39'4 

8 

4'5 

36-1 

8 

4'2 

33  '5 

9 

5*9 

45  *° 

9 

47  :  42-3 

9 

4*3 

,^7 

9 

4-0 

36*0 

10 

4'8 

48-0 

10 

4*5 

45-i 

10 

4'i 

4I-3 

10 

3'8 

38-4 

ii 

4-6 

51*0 

ii 

4  '4 

47'9 

ii 

4-0 

4.V8 

ii 

37 

40-8 

12 

4'5 

5.V9 

12 

4-2 

50-6 

12 

3'9 

46-3 

12 

3-6 

43'i 

13 

4'4 

56-4 

13          4'I 

53'3 

J3 

3-8 

48-8 

13 

3  '5 

45  '4 

H 

4-2 

59'4 

14      4-0  !  55-8 

14 

37 

Si-i 

14 

3'4 

47'5 

15 

4-2 

62-3 

15 

3'9    SB'S 

15 

3'6 

.S3  "6 

IS 

3'3 

49  -8 

16 

4'i 

65-0 

16 

3-8 

6i'i 

16 

3'5 

56-0 

16 

3'3 

53  >0 

17 

4-0 

67-8 

17 

37 

63-6 

17 

3  "4 

58-3 

17 

3'2 

S4'2 

18 

3'9 

70-4 

18 

37 

66-1 

18 

V4 

6o-s 

18 

3-1 

56'3 

19 

3-8 

74'3 

19 

3-6 

68-6 

19 

3'3 

62-8 

19 

3'i 

58-4 

58  Quantitative  Chemical  Analysis. 

By  taking  the  logarithm  of  formula  (i)  we  obtain 
log  a 

- 


the  logarithms  being  common  logarithms,  and  this  formula 
enables  us  to  find  the  least  number  of  washings  requisite  to 

bring   down   the  impurity  to  a   fraction    •     of  its  original 

& 

amount,  by  the  use  of  a  quantity  v  at  each  washing. 
The  foregoing  table  has  been  calculated  from  it.  The  top- 

most line  of  the  heading  shows  the  fraction  i. 

a 

By  employing  for  each  treatment  the  same  volume  of  wash- 
water,  and  approximately  determining  the  relative  volumes 
of  the  precipitate,  and  of  the  washing  liquid,  used  each  time, 
we  may  obtain  from  the  table  on  the  preceding  page,  calculated 
from  the  foregoing  formula,  the  minimum  number  of  treat- 
ments required  to  reduce  the  impurity  in  the  precipitate  to 
•unnnro  *7mro  *WOT  or  TOTTO  a  ^  its  weight.  Column  I.  gives 
the  relation  of  the  volume  of  the  precipitate  to  the  volume  of 
the  washing-fluid  employed  for  each  treatment.  Column  II., 
the  minimum  number  of  treatments  necessary  for  the  par- 
ticular extent  of  washing  desired,  and  Column  III.,  the  total 
volume  of  wash-water  which  will  be  obtained.  (See  p.  57.) 

Let  us  suppose  that  we  have  a  precipitate  occupying  a  volume 
at  the  bottom  of  the  beaker  of  thirty  cubic  centimetres,  and 
that  the  amount  of  liquid  we  employ  to  wash  it  each  time 

is  fifteen  cubic  centimetres,  then    -  is   of  course   0*5,  and 

v 

if  we  wished  to  remove  the  impurities  to  the  yffjwth  part, 
we  learn  from  Column  II.  of  the  table  that  we  must  treat  it  at 
least  27  ^267)  times  with  this  amount  of  water  (viz.,  15  c.c.)  — 
that  is  to  say,  the  minimum  amount  of  wash-  water  needed  will 
be  399  cubic  centimetres.  In  cases  of  simple  decantation  from 
beakers,  the  volume  occupied  by  the  precipitate,  as  compared 
with  the  fluid  above  it,  may  be  very  easily  determined  by 
laying  a  strip  of  paper  along  the  side  of  the  beaker,  marking 
off  the  height  of  the  precipitate,  and  level  of  the  liquid,  and 


Filtration. 


finding  the  number  of  times  the  length  corresponding  to  the 
height  of  the  precipitate  may  be  folded  into  the  length  corre- 
sponding to  the  depth  of  the  supernatant  liquid.  In  applying 
this  table  to  niters,  the  capacity  c  of  these  must  be  calculated ; 
it  is  given  by  the  formula 


where  r  is  the  radius  of  the  filter-paper. 

The  following  table  shows  the  capacity  in  cubic  centi- 
metres of  various  filters,  placed  in  a  funnel  whose  oppo- 
site sides  form  an  angle  of  60°. 


No.  3 
»   4 

6-i  c.c. 
I4-5    „ 

No.  5 
„    6 

28-3  c.c.      |  No.  7 

49-0    „       ••;,,  '8 

i 

77-8  c.c. 
116-1    ,, 

When  the  whole  of  the  precipitate  has  been  brought  on  to 
the  filter,  the  unoccupied  volume  of  the  latter  is  determined 
by  filling  it  with  water  from  a  burette.  If  w  be  the  amount 

of  water  required  to  fill  it,  then  gives  the  entry  -  in 

Col.  I.  of  the  table  on  p.  57,  whence  we  obtain  the  minimum 
number  of  washings  required.* 

The  rapidity  with  which  a  liquid  filters  is  proportional  to 
the  difference  of  pressure  exerted  on  its  upper  and  lower 
surfaces.  By  the  ordinary  method  of  filtration  this  differ- 
ence seldom  exceeds  six  millimetres  of  mercury.  By  increas- 
ing the  difference  we  accelerate  one  of  the  most  tedious  of 
the  operations  of  quantitative  analysis.  The  following 
arrangement  effects  this  acceleration  to  the  desired  extent. 
A  glass  funnel  is  chosen  of  about  8  centimetres  in  depth, 
the  sides  of  which  are  free  from  irregularities  or  bulgings, 
and  subtend  an  angle  of  60°.  The  stem  should  be  long  and 

*  Bunsen,  Ann.  der  Chem.  u.  Pharm.,  vol.  cxlviii.  p.  269.  In  the 
absence  of  exact  knowledge  respecting  the  nature  of  precipitates,  whether 
pervious  or  impervious  to  liquids,  and  in  what  degree,  or  whether 
different  liquids  have  different  powers  of  adhering  to  or  penetrating 
precipitates,  we  must  regard  the  above  process  as  an  attempt  only  to 
place  the  operation  of  washing  upon  a  quantitative  basis. 


6o 


Quantitative  C/icmical  Analysis. 


narrow,  and  the  end  should  be  cut  obliquely.  A  small 
cone,  i  to  i  J  centimetre  in  depth,  of  thin  platinum  foil  or 
gauze,  and  having  exactly  the  angle  of  the  funnel,  is  dropped 
into  the  apex,  and  over  it  is  fitted  the  filter,  with  all  the  prer 
cautions  described  on  p.  53.  The  following  is  the  readiest 
method  of  obtaining  the  platinum  cone  of  the  desired 
shape.  A  circular  piece  of  writing  paper,  10  or  12  centi- 
metres in  diameter,  is  folded  like  a  filter,  and  placed  in  the 
funnel  so  as  to  fit  accurately  to  its  sides,  especially  near  its 
apex.  It  is  kept  in  position  by  a  few  drops  of  sealing  wax, 
and  is  saturated  with  oil  by  means  of  a  feather,  care  being 
taken  that  no  drops  of  the  oil  remain  at  the  point  of  the 
paper  cone.  A  thin  cream  of  plaster  of  Paris  is  then  poured 
into  the  paper  mould,  and  a  small  handle  is  inserted  into  it 
just  before  the  mass  becomes  solid.  In  a  few  hours  the 
plaster  cast  will  be  dry  enough  to  be  removed  from  the 
funnel,  together  with  the  oiled  paper.  The  outside  of  the 
paper  is  now  thoroughly  oiled,  and  inserted  into  a  small 
crucible,  or  similarly  shaped  vessel,  of  4  or  5  centimetres  in 
height,  filled  with  cream  of  plaster  of  Paris.  As  soon  as  the 
outer  mould  is  dry,  the  plaster  cone  is  removed,  and  the  paper 
rubbed  off  it.  In  this  manner  a  solid  cone,  fitting  into  a  hollow 
cone,  is  obtained,  both  of  which  possess  exactly  the  angle  of 
FIG.  25.  inclination  of  the  fun- 

nel (fig.  25).  Apiece 
of  platinum  foil,  of 
such  thickness  that 
i  square  centimetre 
weighs  0-15  gram,  is 
cut  into  the  shape  and 
size  represented  in 

fig.  26  :  it  is  divided  by  a  pair  of  scissors 
along  the  line  a  b,  as  far  as  the  centre  a. 
The  foil  is  then  held  in  the  flame  of  the  lamp  for  a  few 
minutes,  to  render  it  pliable,  and  placed  against  the  plaster 
cone,  so  that  the  point  a  is  at  the  end  of  the  cone ;  the  side 
a  b  d  is  folded  against  the  cone,  and  over  this  is  folded  the 


FIG.  26. 


Filtration. 


61 


remainder,  a  b  c,  so  that  the  foil  also  becomes  a  cone,  the 
sides  of  which  have  exactly  the  same  inclination  as  those  of 
the  plaster  cast,  and  also  of  the  funnel.  The  shape  of  the 
platinum  funnel  may  be  completed  by  dropping  it  into  the 


FIG. 


FIG.  27. 


hollow  mould,  and  pressing  it  down  by  means  of  the  plaster 
cone :  this  shape  of  course  may,  at  any  time,  be  again  given  to  it 
by  simple  pressure  between  the  two  cones.  The  platinum  cone 
should  allow  no  light  to  pass  through  its  apex  :  when  pro- 
perly made  it  will  support  a  filter  filled  with  liquid,  under  a 
pressure  of  an  atmosphere,  without  the  paper  breaking :  the 


62  Quantitative  Chemical  Analysis. 

small  space  between  the  folds  of  the  foil  is  quite  sufficient  to 
allow  of  the  passage  of  a  rapid  stream  of  water  from  the  filter. 
The  stem  of  the  funnel  is  pushed  through  a  caoutchouc 
cork,  pierced  with  two  holes,  and  fitting  into  a  thick  glass 
flask  (A,  fig.  27);  the  second  hole  carries  a  piece  of  glass 
tube  ending  immediately  under  the  cork  and  leading  to  the 
instrument  which  creates  the  difference  in  pressure.  This 
is  also  seen  in  fig.  27.  A  brass  tube,  a  a,  about  i 
metre  in  length  and  8  millimetres  in  internal  diameter,  has 
its  upper  end  cut  obliquely  in  the  manner  seen  in  fig.  2  7 A. 
At  about  5  centimetres  from  the  end  is  a  side  tube  c  of 
equal  diameter  and  5  centimetres  long,  into  which  is  screwed 
a  short  piece  of  tube  d\  the  ends  of  this  tube  */are  fitted 
with  narrow  brass  tubes,  e  and/,  4  millimetres  in  diameter 
and  2  centimetres  long.  Over  f  is  pushed  a  piece  of  thick 
caoutchouc  tube  4  centimetres  in  length.  This  tube  must 
be  made  of  good  caoutchouc  :  it  should  be  about  6  milli- 
metres in  external  diameter,  and  its  bore  should  not  exceed 
2  millimetres  in  width.  Before  introducing  it  into  <r,  a  piece 
of  wood,  somewhat  wider  than  its  bore,  is  pushed  into  it, 
and  the  caoutchouc  is  cut  through  to  the  wood  by  a  smart 
blow  on  the  head  of  a  chisel,  2  centimetres  broad,  placed 
against  the  tube  at  15  millimetres  from  the  end.  The  wood 
is  then  withdrawn,  and  the  end  of  the  caoutchouc  tube  is 
stopped  air-tight  by  a  short  length  (i  centimetre)  of  glass 
rod,  held  firmly  in  position  by  binding  wire.  The  thick 
caoutchouc  tube  so  cut,  forms  a  very  efficient  valve,  which, 
on  the  application  of  pressure  from  within,  opens,  but  closes 
immediately  by  outward  pressure.  The  tube  being  of  con- 
siderable thickness  in  the  walls,  is  rigid,  and  does  not  collapse 
even  under  a  pressure  of  an  extra  atmosphere.  The  upper 
end  of  the  tube  a  a  is  connected  by  means  of  a  short  piece 
of  elastic  caoutchouc  tubing  with  the  water-supply;  this 
tube  should  be  bound  round  with  calico  to  within  5  or  6 
centimetres  of  the  end  near  the  brass  tube,  since  it  will  be 
subject  to  considerable  inward  pressure.  On  allowing  a 
sufficient  amount  of  water  to  flow  through,  it  commences 
to  pulsate  as  the  india-rubber  valve  intermittently  opens  and 


Filtration.  63 

shuts.  Rapid  suction  is  thus  set  up,  and  the  instrument 
exhausts  a  closed  vessel  in  a  comparatively  short  time  to 
within  the  pressure  due  to  the  tension  of  aqueous  vapour 
corresponding  to  the  temperature  of  the  water  flowing  down 
the  tube,  plus  the  tension  required  to  open  the  caoutchouc 
valve.  The  degree  of  exhaustion  is  determined  from  the 
height  of  the  mercury  in  the  manometer  ;;z,  which  is  con- 
nected with  the  tube  d  by  means  of  a  piece  of  strong  caout- 
chouc tubing.  The  entire  apparatus  is  fixed  upon  a  board, 
which  may  have  a  foot  if  it  is  desired  to  move  it  from  place 
to  place  in  the  laboratory  ;  or  it  may  be  fixed  in  a  position 
where  the  water  can  most  conveniently  flow  away.*  By 
connecting  the  tube  h  with  the  flask  holding  the  funnel  (or 
with  an  intermediate  vessel  to  which  several  flasks  are 
attached)  we  diminish  the  pressure  to  which  the  under  sur- 
face of  the  liquid  to  be  filtered  is  exposed,  so  that  the  filtrate 
is  driven  with  greatly  increased  rapidity  through  the  pores  of 
the  paper  ;  the  filter  itself  is  prevented  from  being  pushed 
through  into  the  stem  by  the  closely-fitting  little  platinum 
cone  which  supports  it. 

The  diminution  of  pressure  may  also  be  readily  brought 
about  by  the  aid  of  the  little  apparatus  seen  in  fig.  276,  which 
is  specially  applicable  to  water  Fig.  27,  B. 

under    high    pressure.      The  2- 

apparatus   is   attached  by  a 


stout  piece  of  caoutchouc  tub-      ,u    d  /, ; 
ing  to    the    water  tap;    the     '* 


water  flowing  in  in  the  di- 
rection indicated  by  the  ar- 
rows. When  forced  through 
the  narrow  internal  tube  b 
into  the  sharply  bent  fall- 
tube  c  c,  a  partial  vacuum  is 
created  in  the  bulb-shaped  portion,  d,  and  hence  within  the 

*  For  an  explanation  of  the  principle  of  this  apparatus,  see  a  paper 
by  Mendelejeff,  Kirpitschoff,  and  Schmidt,  Ann.  der  Chem.  u.  Pharm., 
January  1873.  See  also  Jagn,  Ann.  der  Chem.  u.  Pharm.,  Feb.  1873. 


64  Quantitative  Chemical  Analysis. 

filter-flask  or  other  vessel  with  which  it  is  connected  by 
the  T-tube  e,  which  may  also  be  put  in  connection,  if  ne- 
cessary, with  .a  manometer.  In  the  tube  /  is  a  small  caout- 
chouc valve,  similar  to  that  described  in  the  apparatus  shown 
in  fig.  2  7  A,  to  prevent  the  possible  reflux  of  the  water. 

To  use  this  apparatus  for  filtering,  the  liquid  resting  over 
a  precipitate  is  cautiously  poured  on  to  the  filter  fitted  to  the 
funnel,  with  the  precautions  detailed  on  p.  53,  the  action  of  ,the 
pump  is  set  up,  and  as  the  liquid  flows  through  into  the  flask 
fresh  portions  are  added  until  the  whole  has  been  decanted. 
The  precipitate  is  then  transferred  in  the  ordinary  manner  and 
washed  by  the  addition  of  water  from  an  open-mouthed  vessel, 
and  not  by  a  jet  from  the  wash-bottle.  The  fluid  in  which 
the  precipitate  was  originally  formed,  together  with  that 
necessary  to  transfer  the  precipitate  to  the  filter,  should  be 
allowed  to  flow  away  completely  before  the  process  of  wash- 
ing is  commenced.  Immediately  after  the  precipitate  is 
drained,  but  before  any  channels  commence  to  form  in  it,  the 
filter  is  to  be  filled  up  with  water,  poured  cautiously  down 
the  side  of  the  funnel.  When  this  wash-water  has  drained 
away,  the  suction  is  continued  until  the  precipitate  is  seen 
to  shrink,  when  the  filter  is  again  filled  up  over  the  edge  and 
to  within  i  centimetre  of  the  brim  of  the  funnel.  This  process 
is  to  be  twice  repeated,  after  which  the  precipitate  may  be 
drained  almost  dry  by  continuing  the  action  of  the  pump 
for  a  few  minutes.  This  method  of  filtration  and  washing 
is  exceedingly  rapid  as  compared  with  the  old  plan,  and 
requires  very  little  wash-water  by  reason  of  the  compression 
which  the  precipitate  suffers.  Thus  a  precipitate  of  chromium 
sesquioxide  weighing  about  0-24  gram,  required  i  hour  48 
minutes,  and  1050  cubic  centimetres  of  water  to  wash  it  to 
within  3~ff^oir  by  tne  °ld  method,  whilst  with  the  new  plan 
the  same  amount  of  sesquioxide  required  only  from  12  to  14 
minutes,  and  from  39  to  41  cubic  centimetres  of  water. 
(Bunsen.) 

A  further  advantage  attending  the  use  of  the  suction 
apparatus  arises  from  the  condition  of  the  precipitates  after 


Drying  and  Igniting  Precipitates.  65 

filtration.  The  chromium  sesquioxide,  for  example,  is  left  so 
dry,  that  without  further  desiccation,  the  precipitate,  wrapped 
in  the  filter,  may  be  placed  in  the  crucible  over  the  lamp, 
and,  after  cautiously  charring  the  paper,  maybe  ignited  without 
any  apprehension  of  loss  by  proj  ection.  Many  other  precipi- 
tates which  experience  no  alteration  when  ignited  in  contact 
with  paper,  such  as  ferric  oxide,  alumina,  &c.,  may  be  treated 
in  the  same  way.  The  paper  being  nearly  dry  may  also  be 
readily  detached  from  the  funnel ;  when  opened  out  on  a 
flat  surface,  the  coherent  precipitate  may  easily  be  removed 
by  means  of  a  thick  platinum  wire,  so  as  scarcely  to  leave  a 
trace  upon  the  filter.  This  ready  method  of  removing  the 
precipitate  is  of  great  value  when  we  have  occasion  to  treat 
it  with  a  solvent  or  flux. 

This  suction  apparatus  may  be  used  for  a  variety  of  pur- 
poses in  addition  to  that  of  filtration ;  it  may  be  employed 
as  an  aspirator  in  quantitative  operations,  since  the  volume 
of  air  passing  through  the  tube  can  be  regulated  with  the 
utmost  nicety  by  the  aid  of  the  screw- clamp  ;  it  may  also  be 
applied  to  the  evacuation  of  desiccators  or  vessels  in  which  the 
concentration  of  liquids  in  vacuo  is  conducted,  to  freeing 
crystals  from  mother-liquors,  &c. 

Drying  the  precipitate.— -In  the  majority  of  cases  it  is  neces- 
sary to  dry  the  precipitate  thoroughly  before  it  can  be  further 
treated  with  the  view  of  determining  its  weight.  The  water 
in  the  stem  of  the  funnel  is  removed  by  filter-paper,  and  the 
mouth  of  the  funnel  closed,  to  protect  the  precipitate  from 
dust,  by  placing  a  moistened  filter  over  it ;  on  drying,  the 
paper  adheres  to  the  rim  and  makes  a  very  efficient  cover. 
The  funnel  is  then  placed  in  the  steam-bath  represented  in 
fig.  13  (p.  38),  and  kept  there  until  the  paper  and  precipitate 
are  completely  dried.  This  method  of  drying  the  precipitate 
is  preferable  to  that  of  supporting  the  funnel  directly  over 
the  lamp,  for  in  addition  to  the  risk  of  cracking  the  stem, 
the  latter  method  has  the  further  disadvantage  of  causing 
the  precipitate,  by  reason  of  the  manner  of  heating,  to  adhere 


66  Quantitative  Cliemical  Analysis. 

to  the  paper.  When  dried  in  the  steam-bath,  the  precipi- 
tate, in  contracting,  detaches  itself  from  the  filter  ;  so  much 
so,  that  many  curdy  or  gelatinous  precipitates  like  silver 
chloride,  or  ferric  or  chromic  oxides,  may  be  almost  com- 
pletely shaken  out  of  the  funnel  into  the  crucible  in  which 
they  are  to  be  weighed.  This  ready  separation  of  the  dried 
precipitate  from  the  paper  materially  conduces  to  accuracy 
in  determining  its  weight. 

Igniting  and  weighing  the  precipitate. — Since  the  precipitate 
requires  to  be  weighed  in  a  perfectly  dry  state,  it  is  in 
general  necessary  to  remove  it  from  the  filter  and  to  ignite  it. 
A  porcelain  or  platinum  crucible  is  heated,  allowed  to  cool 
in  the  desiccator,  and  weighed.  It  is  then  placed  on  the 
sheet  of  black  glazed  paper,  together  with  the  platinum  wire 
and  feather  (Fig.  21,  p.  51).  The  filter  is  removed  from  the 
funnel,  opened  out,  and  the  detached  fragments  of  the  pre- 
cipitate allowed  to  fall  into  the  crucible.  The  portions  of 
the  precipitate  adhering  to  the  filter  are  loosened  by  rubbing 
its  sides  together,  care  being  taken  that  its  surface  is  not 
thereby  destroyed,  otherwise  filaments  of  paper  are  apt  to 
contaminate  the  precipitate ;  these  may  either  escape  burn- 
ing in  the  subsequent  ignition,  or  if  burnt,  may  alter  the 
composition  of  the  precipitate.  The  detached  precipitate 
is  then  added  to  the  main  quantity  already  in  the  crucible. 
Care  must  be  taken  to  remove  as  much  of  the  precipitate 
as  possible  from  the  filter ;  but  however  carefully  the  opera- 
tion may  be  performed,  a  considerable  amount  of  the  sub- 
stance invariably  remains,  either  on  the  surface  of  the  paper 
or  contained  within  its  pores.  When  the  substance  suffers 
no  change  by  ignition  with  carbonaceous  matter,  it  may  be 
recovered  by  burning  the  paper,  and  adding  the  ash  to  the 
crucible.  The  known  weight  of  the  filter-ash  is  then  sub- 
tracted from  the  increase  in  the  weight  of  the  crucible. 
The  filter  is  burnt  in  the  manner  described  on  p.  51.  The 
paper  should  be  so  folded  that  the  soiled  half  of  the  filter  is 


Weighing  Precipitates.  67 

in  the  centre  :  there  is  thus  less  chance  of  loss  from  projection 
or  from  the  precipitate  fusing  to  the  heated  platinum  wire. 
The  ash  is  shaken  into  the  crucible,  which  is  then  ignited,  at 
first  gently,  and  with  the  lid  on ;  afterwards  more  strongly,  and 
with  the  lid  removed.  The  degree  and  duration  of  the  heat 
depend,  of  course,  on  the  nature  and  amount  of  the  pre- 
cipitate :  as  a  general  rule  from  five  to  ten  minutes  at  a  low 
red  heat  will  be  sufficient.  The  crucible  is  then  placed  in 
the  desiccator,  and  weighed  when  cold.  It  must  be  heated 
a  second  time  and  again  weighed,  to  ascertain  t*hat  its 
weight  is  constant. 

Some  precipitates  suffer  change  when  ignited  in  contact 
with  carbonaceous  matter,  or  become  altered  in  composition 
at  the  high  temperature  necessary  to  burn  the  carbon  com- 
pletely. Thus  silver  chloride  becomes  reduced  to  metallic 
silver  in  contact  with  carbon,  and  calcium  carbonate  is 
converted  into  caustic  lime  at  a  red  heat.  In  weighing 
silver  chloride,  for  example,  the  precipitate  is  detached  as 
far  as  practicable  from  the  filter,  and  the  crucible  in  which 
it  is  placed  is  gently  heated  until  the  chloride  fuses,  and 
when  cold  it  is  weighed.  The  paper  is  now  folded  in  the 
usual  way,  the  soiled  portion  being  in  the  centre,  and  it  is 
burned  in  the  manner  described,  and  the  ash  added  to  the 
fused  silver  chloride.  The  crucible  is  again  weighed  :  its 
increase  of  weight  gives  the  amount  of  the  filter-ash,  together 
with  the  quantity  of  metallic  silver  which  has  been  reduced 
from  the  state  of  silver  chloride  by  contact  with  ignited  car- 
bonaceous matter.  Since  108  parts  of  silver  correspond  to 
143-5  of  silver  chloride,  the  amount  of  silver  chloride  in  the 
pores  of  the  paper  can  be  readily  calculated  from  this 
reduced  silver  :  it  is  of  course  added  to  the  weight  of  the 
main  quantity  of  the  chloride.  The  cases  in  which  it  is 
necessary  to  weigh  the  filter-ash  separately  will  be  mentioned 
as  they  occur. 

Whenever  practicable,  a  platinum  crucible  should  be  em- 
ployed on  account  of  the  readiness  with  which  it  may  be 

F  2 


'68  Quantitative  Chemical  Analysis. 

heated  to  redness.  Indeed,  in  some  cases,  its  use  is  almost 
indispensable,  as  in  the  conversion  of  calcium  oxalate  into 
carbonate,  and  of  magnesium-ammonium-phosphate  into 
magnesium  pyrophosphate.  Platinum  vessels,  however, 
cannot  be  used  for  the  ignition  of  silver  chloride  or  bromide 
FlG  28  or  of  lead  chloride.  Many  oxides, 

^  sulphides,  and  phosphides  cannot 
be  heated  in  contact  with  pla- 
tinum without  injury  to  the  metal. 
After  prolonged  ignition  over  a 
lamp,  especially  if  the  reducing 
portion  of  the  flame  be  permitted 
to  impinge  upon  it,  the  lower  por- 
tion of  the  crucible  loses  its  lustre 
and  appears  to  be  superficially  corroded.  This  appearance 
is  said  to  be  due  to  the  formation  of  a  carbide  of  platinum. 
Red-hot  platinum  crucibles  should  never  be  touched  with 
brass  tongs  or  placed  in  brass  rings,  as  black  stains  are  thus 
formed  on  the  metal.  They  are  best  heated  on  pipe-clay 
triangles  or  on  thin  platinum  triangles  supported  on  a  triangle 
of  stout  iron  wire,  fig.  28.  Clean  iron  tongs  will  be  found 
more  generally  convenient  than  brass  tongs.  Platinum 
crucibles  may  be  cleaned  by  scouring  with  moist  sea-sand, 
which  does  not  scratch  the  metal ;  stains  which  cannot  thus 
be  removed  are  got  rid  of  by  heating  with  acid  potassium 
sulphate,  or  borax,  the  crucible  being  afterwards  thoroughly 
washed  with  hot  water  and  scoured  with  sea-sand. 

Collection  of  precipitates  on  weighed  filters. — Occasionally 
we  have  to  deal  with  a  precipitate  which  cannot  be  ignited  to 
ensure  the  expulsion  of  moisture  before  being  weighed.  The 
precipitate  must  be  weighed  therefore  on  the  filter  on  which 
it  is  collected.  Accordingly  the  weight  of  the  filter  itself 
must  be  known.  The  paper  is  folded  in  the  usual  manner, 
placed  in  a  stoppered  tube,  or  between  well-fitting  watch- 
glasses  pressed  together  by  a  clip,  and  heated  in  the 
steam-chamber  for  an  hour  or  so.  The  stoppered  tube, 


Weighing  Precipitates.  69 

or  watch-glasses,  together  with  the  filter-paper,  are  allowed 
to  cool  in  the  desiccator,  and  weighed  when  cold.  The 
filter  is  then  fitted  into  the  funnel,  and  the  precipitate  is 
brought  on  to  it,  the  tube  or  the  watch-glasses  being  mean- 
while left  in  the  balance-case.  The  paper  and  precipitate 
are  first  dried  in  the  funnel,  the  filter  is  then  detached  from 
the  glass,  and  placed  in  the  tube  or  between  the  watch- 
glasses,  heated  for  some  hours  in  the  bath,  and  repeatedly 
weighed  until  the  weight  is  constant. 

Another  plan  is  to  weigh  two  filters  of  equal  size  (A  and 
B)  against  each  other,  and  mark  the  difference  in  weight  on 
B.  The  precipitate  is  collected  on  A,  the  filtrate  and  wash- 
ings being  allowed  to  pass  through  B  ;  both  are  dried  and 
weighed  against  each  other,  and  the  original  difference  in 
weight  allowed  for. 


70  Quantitative  Chemical  Analysis. 

Q 

PART     II. 
SIMPLE  GRAVIMETRIC   ANALYSIS. 

I.  COPPER  SULPHATE.     CuSO4  +  5H2O. 

Preparation.—^  order  to  obtain  this  salt  in  a  fit  state  for 
analysis,  it  is  necessary  to  purify  it  by  recrystallisation.  The 
blue  vitriol  of  commerce  not  unfrequently  contains  ferrous 
sulphate,  which  cannot  be  removed  even  by  repeated  crystal- 
lisation, as  the  two  sulphates  tend  to  crystallise  together. 
By  heating  the  solution  with  a  few  drops  of  nitric  acid,  the 
ferrous  salt  is  oxidised  to  ferric  sulphate,  and  on  concentrat- 
ing the  liquid,  crystals  of  pure  copper  sulphate  are  easily 
obtained.  Two  hundred  grams  of  clean,  well-developed 
crystals  of  the  commercial  salt  are*  dissolved  in  about  a 
quarter  of  a  litre  of  hot  water,  a  few  drops  of  nitric  acid  are 
added,  the  solution  is  filtered,  if  necessary,  and  boiled  for 
half-an-hour ;  on  cooling  the  liquid  deposits  crystals  of  the 
pure  salt.  After  standing  for  a  few  hours  the  solution  is 
poured  off,  and  the  mother-liquor  is  drained  as  far  as 
possible  from  the  crystals.  The  crystalline  mass  is  broken 
up  by  means  of  a  glass  rod,  and  dried  by  pressure  between 
folds  of  filter-paper.  It  is  advisable  not  to  use  too  great  a 
degree  of  force  in  pressing  the  salt,  as  the  sulphate  may 
thus  become  mixed  with  filaments  of  filter-paper,  which 
interfere  with  the  accuracy  of  the  analytical  operations. 
When  the  greater  portion  of  the  moisture  has  been  removed 
by  repeated  pressure  between  filter-paper,  the  salt  is  wrapped 
in  a  fresh  sheet  of  dry  paper,  and  the  folds  are  placed 
under  a  heavy  weight  for  an  hour  or  two.  Whilst  the  salt  is 
drying,  the  apparatus  required  for  its  analysis  is  got  ready. 
Two  small  thin  test-tubes,  to  hold  about  6  or  8  grams  of  the 
salt,  are  cleaned  and  dried,  and  fitted  with  good,  clean,  soft 


Copper  Sulphate.  71 

corks.  A  couple  of  beakers  of  250  cubic  centimetres  capacity, 
and  two  large  watch-glasses  to  cover  their  mouths,  a  filter- 
flask  fitted  with  its  funnel,  and  two  thin  glass  rods,  all  per. 
fectly  clean  and  dry,  are  also  needed. 

The  complete  analysis  of  copper  sulphate  necessitates 
the  determination  (i)  of  the  water  of  combination  \  (2)  of 
the  copper ;  and  (3)  of  the  sulphuric  acid. 

i.  Determination  of  the  water  of  combination. — Copper  sul- 
phate gives  up  the  whole  of  its  combined  water  on  heating ; 
4  molecules  being  readily  expelled  at  100-110°,  and  the 
remaining  molecule  at  about  200°.  The  determination  of 
the  amount  of  water  expelled  at  different  temperatures 
may  be  made  by  means  of  the  air-bath  (fig.  14,  p.  39),  or 
in  the  apparatus  represented  in  fig.  29.  The  test-tube 
(a)  contains  the  tube  and  salt  to  be  dried;  it  is  about 
8  centimetres  long,  and  2  centimetres  wide ;  into  it  is 
placed  the  narrower  and  shorter  tube,  containing  the  weighed 
amount  of  salt  :  the  tube  a  is  closed  with  a  cork  pierced 
with  two  holes,  into  which  are  fitted  narrow  glass  tubes 
bent  at  right  angles  :  one  of  these  tubes  passes  nearly  to  the 
bottom  of  the  test-tube.  The  narrow  tubes  are  connected 
by  means  of  caoutchouc  tubing  with  the  small  flasks  c  and  dy 
containing  strong  sulphuric  acid  :  the  tube  e  of  the  flask  d 
is  in  connection  with  the  filter-pump,  by  means  of  which  a 
current  of  air,  dried  by  aspiration  through  the  acid  in  the 
flask  c,  is  drawn  over  the  salt.  The  test-tube  dips  into  a 
small  quantity  of  oil  contained  in  a  beaker  of  400  cubic 
centimetres  capacity.  The  tube  is  held  by  means  of  a  clamp 
attached  to  a  retort- stand.  The  oil  is  heated  by  a  small 
flame,  and  the  temperature  is  •  ascertained  by  a  thermometer 
placed  near  to  the  tube. 

By  the  time  this  piece  of  apparatus  has  been  fitted  up, 
and  the  beakers,  &c.,  are  cleaned,  the  copper  sulphate 
under  the  weight  will  be  dry.  One  of  the  test-tubes  which  has 
been  fitted  with  a  cork  is  nearly  filled  with  the  dried  salt ;  any 


72  Quantitative  Chemical  Analysis. 

adhering  powder  is  wiped  from  the  upper  portion  and  edge 
of  the  tube,  and  the  cork  is  replaced.  The  remainder  of 
the  copper  sulphate  is  set  aside  in  a  stoppered  bottle  ;  it  will 
be  useful  for  subsequent  analytical  operations. 

Weigh  the  other  test-tube  and  cork,  and  introduce  about 

FIG.  29. 


i  -5 — 2  grams  of  the  copper  salt,  taking  care  not  to  soil  the 
edge  of  the  tube  ;  replace  the  cork  and  weigh  again.  The 
increase  of  weight  gives  the  amount  of  salt  taken  for  the  deter- 
mination. Take  out  the  cork,  and  leave  it  in  the  balance 
case.  Drop  the  little  tube  containing  the  salt  into  the  wider 
test-tube  of  the  drying  apparatus,  insert  the  cork  and  bent 
tubes,heat  the  oil-bath  to  ioo-no°,with  frequent  stirring,  and 


Copper  Sulphate.  73 

aspirate  a  gentle  current  of  air  through  the  sulphuric  acid. 
In  about  an  hour  the  greater  portion  of  the  water  will  have 
been  expelled:  the  little  tube  is  withdrawn  from  the  wider  one 
by  means  of  the  forceps,  allowed  to  cool  in  the  desiccator, 
and  carried  to  the  balance-room.  When  cold,  the  cork  in 
the  balance-case  is  inserted  into  the  tube,  and  the  loss  of 
water  which  the  sulphate  has  suffered  is  determined  by  a 
second  weighing.  The  cork  is  once  more  withdrawn,  left  in 
the  balance-case,  and  the  tube  again  heated  in  the  oil- 
bath  to  1 00-110°,  a  current  of  dry  air  being  swept  over  it 
for  about  half-an-hour,  after  which  it  is  again  weighed  when 
cold,  in  order  to  determine  if  it  has  parted  with  an  addi- 
tional quantity  of  water.  If  the  second  weighing  is  within 
0*00 1 o  gram  of  the  first,  the  loss  may  be  set  down  as 
constant;  but  if  the  weighings  differ  by  more  than  this 
amount,  the  tube  and  salt  must  be  again  heated  for  half-an- 
hour,  and  weighed  a  third  time,  the  process  being  repeated 
until  a  constant  weight  is  obtained. 

The  temperature  of  the  oil-bath  is  next  raised  to  200°, 
and  maintained  at  this  point  for  about  an  hour,  a 
current  of  dry  air  being  passed  uninterruptedly  through  the 
apparatus.  After  cooling,  the  loss  of  weight  experienced  by 
the  salt  (which  is  now  nearly  white)  is  again  determined, 
the  tube  is  once  more  replaced  in  the  bath,  again  heated, 
and  again  weighed,  the  operation  being  repeated  until  no 
further  loss  of  water  is  perceptible. 

Arrange  the  results  of  the  several  weighings  in  the  follow- 
ing manner  in  your  note-book,  The  numbers  here  given 
are  the  results  of  an  actual  determination. 

ANALYSIS  OF  COPPER  SULPHATE. 
(Date.)          i.  Determination  of  Water. 

Tube  +  cork  +  salt        6  '3400  grams. 
Tube  +  cork        4  -8905      , , 

Weight  of  salt  taken         i  -4495 


74  Quantitative  Chemical  Analysis. 

Weight  of  tube  +  cork  +  salt  after  drying  : 

After  i  hour  at  105° -112°  5'934O  grams 

After  30  min.  additional :   no0  5  -9210  ,, 

io7°-no°  5-9180  „ 

io7°-ii2°  5-9176  ,, 

Water  expelled  at  1 10°  0-4224  ,, 
or  29*14  per  cent. 

After  drying  for  i  hour  at  I9O°-2OO°     5-8250      ,, 
After  further  drying  for  half  an  hour  at  200°     5*8183      ,, 

205°     5-8176      „ 

Additional  loss  of  water    O'looo      ,, 
or  6 '9 1  per  cent. 

2.  Determination  of  the  Copper. — Whilst  the  salt  is  drying 
in  the  oil-  or  air-bath,  proceed  with  the  estimation  of  the 
copper.  This  is  effected  by  precipitating  it  as  cupric  oxide, 
by  the  addition  of  caustic  soda  solution. 

CuSO4  +   2NaHO     =     CuO  +  Na2SO4  +  H2O. 

The  corked  tube  containing  the  copper  sulphate  is  care- 
fully weighed,  and  about  a  gram  of  the  salt  is  shaken  out 
into  one  of  the  clean  and  dry  beakers.  On  replacing  the 
cork,  and  again  weighing  the  tube,  its  loss  of  weight  gives 
the  exact  amount  taken  for  analysis.  Care  must,  of  course, 
be  taken  that  all  the  copper  sulphate  removed  from  the 
tube  finds  its  way  into  the  beaker.  The  salt  is  then  dis- 
solved in  about  50  cubic  centimetres  of  hot  water :  the 
solution  should  be  perfectly  clear,  and  free  from  suspended 
matter.  It  is  boiled,  the  mouth  of  the  beaker  being  mean- 
while closed  by  one  of  the  large  watch-glasses,  in  order  to 
prevent  the  projection  of  any  of  the  solution  on  ebullition. 
The  lamp  is  drawn  aside,  and  a  clear  solution  of  caustic  soda 
is  poured  into  the  liquid,  drop  by  drop,  down  the  side  of  the 
beaker,  the  liquid  meanwhile  being  kept  in  agitation.  A  preci- 
pitate is  at  once  formed  ;  it  is  at  first  of  a  bluish  green  colour, 
but  it  rapidly  darkens  as  it  falls  through  the  hot  liquid,  and 
ultimately  becomes  nearly  black.  These  changes  of  colour 
are  due  to  the  passage  of  the  copper  oxide  from  the 
hydrated  to  the  anhydrous  condition.  The  precipitate  is 


Copper  Sulphate.  75 

allowed  to  settle,  when,  if  sufficient  soda  has  been  added, 
the  liquid  will  be  colourless.  Ascertain  that  the  alkali 
is  in  excess  by  testing  the  solution  with  a  slip  of  red- 
dened litmus-paper.  Fold  a  No.  5  filter,  drop  it  into  the 
platinum  cone,  moisten  thoroughly  with  hot  water,  and  fit  it 
carefully  to  the  funnel  in  the  manner  already  described 
(see  p.  53).  Next  slightly  grease  the  edge  of  the  beaker,  and 
by  means  of  the  glass  rod  decant  the  clear  liquid  on  to  the 
filter  (taking  care  not  to  disturb  the  precipitate  of  copper 
oxide)  and  set  the  pump  in  operation.  When  the  whole  of 
the  liquid  has  been  decanted  on  to  the  filter,  pour  about 
30  or  40  cubic  centimetres  of  hot  water  over  the  precipitated 
oxide,  boil  for  a  few  minutes  with  the  glass  cover  on,  allow 
to  settle,  and  again  pour  the  supernatant  liquid  on  the  filter. 
Repeat  the  washing  by  decantation  and  then  carefully  rinse 
every  particle  of  the  precipitate  with  hot  water  on  to  the  filter. 
It  may  happen  that  a  small  quantity  of  copper  oxide  obsti- 
nately adheres  to  the  beaker  and  cannot  be  removed  by 
washing.  Pour  about  2  cubic  centimetres  of  hot  water  into 
the  beaker,  and  a  couple  of  drops  of  nitric  acid  :  by  the  aid 
of  the  glass  rod,  the  dilute  acid  solution  may  be  made  to 
dissolve  the  adhering  oxide.  This  is  reprecipitated  by  the 
addition  of  a  few  drops  of  soda  solution,  and  thrown  on  to  the 
filter.  Now  fill  up  the  filter  five  times  with  hot  water,  taking 
care  to  allow  the  whole  of  the  wash-water  to  run  through 
before  a  fresh  addition  is  made.  If  these  instructions  are 
followed  the  precipitate  will  be  thoroughly  washed.  The 
funnel  is  withdrawn  from  the  flask,  its  mouth  is  covered  with 
paper  to  protect  the  copper  oxide  from  dust,  and  the  whole 
is  placed  in  the  drying  chamber.  Whilst  the  precipitate  is 
drying,  clean,  dry  and  heat  a  small  porcelain  crucible  and 
lid  (No.  i  size),  place  them  together  in  the  desiccator,  and 
when  cold,  carefully  weigh  them.  When  the  copper  oxide 
is  dry,  it  is  detached  as  far  as  possible  from  the  filter,  and 
transferred  to  the  weighed  crucible.  The  funnel  is  cleaned, 
if  necessary,  by  rubbing  it  with  the  edge  of  the  paper ; 


76  Quantitative  Chemical  Analysis. 

the  filter  is  burned,  and  the  ash  allowed  to  fall  into 
the  crucible.  One  drop  of  nitric  acid  is  allowed  to  moisten 
the  oxide  and  filter-ash,  and  the  crucible  is  gently  warmed 
until  the  mass  is  dry,  when  the  heat  is  raised  until 
the  bottom  of  the  crucible  is  red  hot.  It  is  allowed  to 
cool  slightly,  and  whilst  still  warm,  is  transferred,  together 
with  its  lid,  to  the  desiccator,  and  when  quite  cold,  again 
weighed.  The  crucible  and  lid  are  once  more  heated, 
and  again  weighed  on  cooling ;  care  should  be  taken  that 
the  reducing  gases  from  the  flame  do  not  find  their  way  into 
the  crucible.  The  second  weighing  ought  not  to  differ  more 
than  0-5  milligram  from  the  first  weight.  If  the  difference 
is  greater,  the  operation  must  be  repeated  until  a  constant 
weight  is  obtained.  The  increase  in  weight  of  the  crucible 
and  lid  gives  the  amount  of  copper  oxide  contained  in  the 
quantity  of  salt  taken  for  analysis  plus  the  ash  of  No.  5  filter. 
The  details  of  an  actual  determination  will  show  how  the 
results  ought  to  appear  in  the  note-book  : 

ANALYSIS  OF  COPPER  SULPHATE. 
2.  Determination  of  Copper  by  precipitation  with  Caustic  Soda. 

(1)  Weight  of  tube  +  salt        .         .   10-6052 

(2)  „  „  _9J»8o5_ 

Salt  taken         .         .     I  '0247 

Weight  of  crucible  +  lid  +  CuO  +  ash  (i)     8 •  1 530 

(2)     8-1527 

Crucible  +  lid         .         .         .     7-8240 

0-3287 

Less  filter-ash  No.  5      0*0023 
0-3264 

Calculation*    ^'i  x  0-3264  x  IOO      log  63-1    j_-8ooo 
79-1x1-0247         log -3264     1-5137 

log  IOO      2-OOOO 
33137 

log  79-1     i -8982 

1-4155 

'     .;  log  I  -O247          'OIO5 

1-4050=25-41  Cu. 


Copper  Sulphate.  77 

3.  Determination  of  the  Sulphuric  acid. — This  is  effected 
by  adding  barium  chloride  to  the  solution  of  the  copper 
salt  and  weighing  the  precipitated  barium  sulphate  : 

CuS04  +  BaCl2     =     BaSO4  +  CuCl2 

About  one  gram  of  the  copper  sulphate  is  weighed  out  into 
the  second  beaker,  and  is  dissolved  in  30-40  cubic  centi- 
metres of  water,  a  few  drops  of  hydrochloric  acid  are  added, 
and  the  solution  is  heated  to  the  boiling  point.  The  lamp 
is  now  drawn  aside  and  solution  of  barium  chloride  is 
added  drop  by  drop.  In  order  to  determine  whether  an 
excess  of  the  precipitant  has  been  added,  the  barium  sul- 
phate is  allowed  to  subside,  and  when  the  liquid  is  sufficiently 
clear,  a  drop  of  the  barium  chloride  solution  is  poured  down 
the  side  of  the  beaker.  If  an  increased  turbidity  ensues,  the 
liquid  is  again  heated  and  a  further  quantity  of  barium 
chloride  added  :  the  precipitate  is  once  more  allowed  to 
settle,  and  the  liquid  again  tested  by  the  cautious  addition  of 
barium  chloride.  When  you  have  assured  yourself  that  the 
precipitation  is  complete,  cover  the  beaker  and  set  it  aside 
in  a  warm  place  for  a  few  hours.  If  you  attempt  to  filter 
the  turbid  liquid  immediately,  the  finely  divided  precipitate 
will  inevitably  pass  through  the  pores  of  the  paper,  but  on 
standing,  especially  after  precipitation  from  a  hot  solution 
slightly  acidified  with  hydrochloric  acid,  the  barium  sulphate 
becomes  denser  and  more  granular.  When  the  precipitate 
has  completely  settled  add  one  more  drop  of  barium  chloride 
to  again  assure  yourself  that  the  precipitation  is  complete, 
and  proceed  to  collect  the  barium  sulphate.  Fit  a  No.  5 
filter  carefully  into  the  funnel,  grease  the  edge  of  the  beaker, 
pour  the  clear  liquid,  by  the  help  of  the  glass  rod,  on  to  the 
filter,  and  cautiously  set  the  pump  working.  When  the  whole 
of  the  liquid  has  passed  through,  pour  40-50  cubic  centi- 
metres of  hot  water  over  the  barium  sulphate  and  boil  the 
liquid  for  a  few  minutes  ;  the  precipitate  is  allowed  to  settle 
and  the  liquid  (which  will  now  be  slightly  turbid)  poured  on 


78  Quantitative  Chemical  Analysis. 

to  the  filter.  It  is  well  to  stop  the  flow  of  water  in  the 
pump  until  the  first  filter-full  of  liquid  has  passed  through, 
otherwise  there  is  danger  of  the  precipitate  making  its  way 
through  the  filter.  When  the  whole  of  the  liquid  on  the 
filter  has  passed  through,  fill  up  the  funnel  again  with  the 
wash-water,  cautiously  set  the  pump  in  operation,  rinse  the 
precipitate  on  to  the  filter  with  hot  water,  remove  any  barium 
sulphate  adhering  to  the  beaker  by  means  of  a  feather,  and 
wash  five  or  six  times  with  hot  water.  Take  care  to  allow 
the  whole  of  the  liquid  to  pass  through  before  a  fresh 
quantity  is  poured  in,  or  the  filtrate  may  become  turbid. 
Dry  the  precipitate  and  transfer  it  from  the  filter  to  a 
weighed  porcelain  crucible,  burn  the  filter  and  add  the 
ash  to  the  crucible.  Place  the  crucible  on  the  triangle 
and  cautiously  heat  it  with  the  lid  on  for  2  or  3  minutes, 
increase  the  heat  and  keep  the  bottom  red  hot  for  5  or 
10  minutes,  allow  to  cool  in  the  desiccator,  and  when  cold, 
weigh  :  repeat  the  heating  and  again  weigh.  The  two 
weighings  ought  to  agree.  The  results  should  thus  appear 
in  the  note-book  : 

ANALYSIS  OF  COPPER  SULPHATE. 

3.  Determination  of  Sulphuric  Acid  by  precipitation  as  Barium 
Sulphate. 

Weight  of  tube  +  salt  (before)     9-5803 
(after)     8-6115 


0-9688 


(1)  Crucible  +  lid  +  BaSO4  +  ash    87317 

(2)  ,,  „  ,,  „  8-7315 

Crucible  +  lid     7-8242 


0-9073 
Less  ash  No.  5       -0023 

0-9050 


Copper  Sulphate.  79 

Calculation:    ?6_>^o -9050x100  log  96  1-9823 

233-1x0-9688  log  -905  1-9566 

log  100  2-0000 


log  233-1     2-3676 

J-57'3 

log  -9688     1-9863 

SO4=     i -5850  =  38 -46  p.  ct 
The  complete  analysis  is  therefore  as  follows  : 

Calculated  Found 

Cu  63-1         25-33  25-41 

so4  96-0      38-54         38-46 

4H2O  expelled  at  ioo°-i  10°    72-0        28-90  29-14 

H2O          ,,  200°          18-0          7-23  6-91 

249-1       loo-oo  99*92 


II.  SODIUM  CHLORIDE  (Na  Cl).    * 

Preparation. — Common  salt  rarely  contains  more  than  98 
per  cent,  of  sodium  chloride,  its  principal  impurities  being 
calcium  sulphate  and  magnesium  chloride.  These  sub- 
stances cannot  be  readily  removed  by  recrystallisation,  but 
by  adding  hydrochloric  acid  to  a  strong  solution  of 
the  salt,  pure  chloride  of  sodium  is  precipitated,  and 
the  magnesium  chloride  and  calcium  sulphate  remain  dis- 
solved. About  70  grams  of  salt  are  dissolved  in  a  quarter 
of  a  litre  of  hot  water,  the  solution  is  filtered  and  saturated 
with  hydrochloric  acid  gas.  The  apparatus  represented  in 
fig.  30  may  be  conveniently  used  for  this  purpose.  The 
flask  a  contains  the  salt  and  sulphuric  acid,  and  the  evolved 
hydrochloric  acid  gas,  after  passing  through  a  small  quantity 
of  the  acid  solution  contained  in  the  bottle  b,  is  led  into  the 
filtered  brine.  The  exit-tube  of  the  apparatus  is  replaced  by 
a  small  funnel  dipping  into  the  solution  of  salt  :  this  method 
of  delivering  the  gas  into  the  liquid  prevents  the  possibility 
of  the  precipitated  sodium  chloride  interfering  with  the  pas- 


So  Quantitative  Chemical  Analysis. 

sage  of  the  gas  by  closing  the  outlet.  The  salt  begins  to 
separate  out  almost  immediately,  and  in  an  hour  or  so  the 
process  may  be  interrupted.  The  liquid  is  poured  from  the 
precipitated  salt,  which  is  washed  once  or  twice  with  pure 
strong  hydrochloric  acid  solution,  allowed  to  drain,  and 
heated  gently  in  a  porcelain  basin.  The  moisture  cannot 

FIG.  30. 


be  removed  by  filter-paper,  as  the  strong  acid  would  cause 
the  contamination  of  the  salt  with  the  iron,  &c.,  contained 
in  the  ash.  The  mass  should  be  heated  gently  in  a  porcelain 
crucible  until  all  the  acid  is  expelled,  powdered  roughly 
while  still  warm,  and  a  portion  introduced  into  a  small  dry 
tube  fitted  with  a  good  cork.  The  remainder  of  the  salt  is 
placed  in  a  stoppered  bottle  :  it  will  prove  useful  in  sub- 
sequent operations. 


Sodium  Chloride.  81 

i.  Determination  of  the  Chlorine. — This  is  effected  by  pre- 
cipitation as  silver  chloride,  by  means  of  silver  nitrate 
solution. 

NaCl  +  AgNO3     =     AgCl  +  NaNO3. 

About  0*5  gram  of  the  salt  is  weighed  out  into  a  beaker  of  80 
cubic  centimetres  capacity  and  dissolved  in  30-40  cubic  centi- 
metres of  water ;  a  few  drops  of  pure  nitric  acid  are  added, 
together  with  a  solution  of  silver  nitrate.  If  sufficient  silver 
solution  has  been  added,  the  chloride  separates  out  as  a  dense 
curdy  precipitate.  When  you  have  satisfied  yourself  that  all  the 
chlorine  is  precipitated,  heat  the  liquid  to  near  the  boiling 
point,  stirring  it  occasionally  by  means  of  a  glass  rod,  and 
allow  the  precipitate  to  settle  by  placing  the  beaker,  protected 
from  the  dust,  in  a  warm  place  for  a  few  hours.  Be  careful 
to  protect  the  silver  chloride  as  much  as  possible  from  the 
light.  Pour  the  clear  liquid  on  to  a  filter,  wash  twice  by 
decantation  with  hot  water,  carefully  rinse  the  precipitate  on 
to  the  filter,  wash  it  5  or  6  times  with  hot  water  and  dry 
it  in  the  steam  bath.  Transfer  the  dried  chloride,  detached 
as  completely  as  possible  from  the  filter,  to  a  weighed 
porcelain  crucible  and  heat  very  gradually,  increasing  the 
temperature  until  the  chloride  begins  to  fuse  at  the  edges  of 
the  mass;  allow  to  cool  in  the  desiccator  and  weigh.  If  the 
chloride  has  been  carefully  protected  from  light  it  will  have 
at  most  but  a  slight  violet  tinge,  and  the  fused  portion  will 
have  the  appearance  of  horn.  The  filter-paper  is  folded  in 
the  manner  described  on  p.  67,  and  burnt,  the  ash  being 
allowed  to  fall  on  to  the  chloride.  The  crucible  and  its 
contents  are  again  weighed :  the  increase  in  weight  gives 
the  amount  of  metallic  silver  originally  adhering  as  silver 
chloride  to  the  filter,  together  with  the  ash  of  the  paper. 
The  known  weight  of  ash  in  the  filter  subtracted  from  the 
total  increase  gives  the  amount  of  reduced  silver ;  this  is 
calculated  to  silver  chloride,  and  the  amount  added  to  the 
main  quantity.  An  actual  example  may  make  this  clearer : 

G 


82  Quantitative  Chemical  Analysis. 

Sodium  Chloride  taken  0*4065  grams 

Crucible  +  AgCl  8-9710     „ 

+  Ag  +  ash  8-9813      „ 

Crucible  7-9860      „ 

8-9813-  8-9710  =  0-0103 
Less  ash    0023 

Ag=oo8o  =  AgCl    o-o  1  06 
8-9710-7-9860—0-9850 
Total  AgCl  =  0-9956 
=  6o.6o        cent 


143  '5  *  0*4065 

The  fused  silver  chloride  may  readily  be  detached  from 
the  crucible  by  placing  over  it  a  small  piece  of  zinc  and 
adding  a  few  drops  of  dilute  sulphuric  acid.  The  semi- 
reduced  mass,  together  with  the  silver  chloride,  precipitated 
by  adding  a  few  drops  of  hydrochloric  acid  to  the  nitrate 
should  be  put  into  a  bottle  labelled  '  silver  residues.'  When 
these  residues  have  sufficiently  accumulated  they  are  to 
be  worked  up  as  directed  in  the  Appendix. 

Determination  of  the  Sodium.  —  The  salt  is  converted  into 
sodium  sulphate  by  the  action  of  strong  sulphuric  acid. 
Clean,  ignite,  and  weigh  a  platinum  crucible  and  lid,  intro- 
duce into  it  0-5  to  i  gram  of  salt,  and  again  weigh.  The 
increase  of  weight  gives  the  amount  of  sodium  chloride  taken. 
Place  the  crucible  on  a  triangle  in  the  slanting  position  repre- 
sented in  fig.  15,  and  add  drop  by  drop  pure  strong  sulphuric 
acid.  Do  not  heat  the  crucible  for  ten  or  fifteen  minutes,  or 
until  the  reaction  has  moderated.  There  is  no  danger  of  any 
of  the  substance  being  lost  by  projection,  if  care  is  taken 
to  place  the  crucible  as  directed,  and  to  add  the  acid  cau- 
tiously. Now  heat  the  crucible  gently  from  the  top,  placing 
the  lid  as  indicated  in  the  figure,  and  allow  the  flame  to 
approach  the  bottom  of  the  crucible  very  gradually.  The 
operation  must  be  done  slowly,  and  with  constant  watching, 


Pearl-ash.  83 

or  a  portion  of  the  sulphate  may  be  lost  by  spirting.  In  a 
few  minutes  the  whole  of  the  hydrochloric  acid  will  have 
been  expelled,  and  dense  fumes  of  sulphuric  acid  will  be 
evolved.  As  these  diminish,  the  heat  is  gradually  raised, 
until  the  crucible  attains  a  full  red  heat.  Maintain  it  at  this 
temperature  for  fifteen  or  twenty  minutes,  put  on  the  lid, 
allow  to  cool,  and  weigh.  Again  heat  to  redness  for  five 
minutes,  and  weigh  a  second  time.  The  operation  is  to  be 
repeated,  until  a  constant  weight  is  obtained.  The  fused 
mass  should  be  quite  white. 

III.    PEARL-ASH. 

Good  pearl-ash  of  commerce  contains  from  $3  to  96 
per  cent,  of  potassium  carbonate,  the  rest  consisting  of 
water,  alkaline  sulphates,  chlorides,  silica,  &c.  To  deter- 
mine its  value  it  is  merely  necessary  to  estimate  the  per- 
centage amount  of  potassium  carbonate  and  water.  The 
quantity  required  to  make  up  106  is  taken  as  the  measure  of 
the  impurities. 

Determination  of  the  Moisture. — Weigh  out  from  3  to  4 
grams  of  the  coarsely-powdered  ash  from  a  tube  into  a  small 
weighed  porcelain  crucible  provided  with  a  lid.  Gently 
heat  the  ash  for  half-an-hour  over  a  small  gas  flame,  and 
weigh  when  cold  ;  again  heat,  and  again  weigh.  It  must 
not  be  forgotten  that  potassium  carbonate  is  highly  hygro- 
scopic ;  the  weighings  must,  therefore,  be  made  as  expedi- 
tiously  as  possible. 

Determination  of  the  Potassium. — The  carbonate  is  con- 
verted into  the  double  chloride  of  platinum  and  potassium 
(PtCl42KCl).  Weigh  out  about  2  grams  of  the  carefully 
sampled  carbonate  and  dissolve  in  30-40  c.c.  of  water.  Filter, 
if  necessary,  into  a  250  c.c.  flask,  wash  the  filter  thoroughly 
from  every  trace  of  alkali,  fill  the  flask  up  to  the  mark  with 
distilled  water,  and  shake  vigorously.  Transfer  two  lots  of 
50  c.c.  each  to  porcelain  basins,  cover  the  basins  with 
glass  plates,  and  add  dilute  solution  of  hydrochloric  acid 

G  2 


84  Quantitative  Chemical  Analysis. 

in  slight  excess.  Heat  the  basins  on  the  water- bath,  and 
when  the  expulsion  of  the  carbonic  acid  is  finished,  rinse 
the  covers  with  a  few  drops  of  hot  water,  and  add  solu- 
tion of  pure  platinum  tetrachloride.  It  is  necessary  to 
add  the  solution  of  platinum  in  considerable  excess ;  about 
i  gram  of  the  metal  is  required  for  0*5  gram  of  the  car- 
bonate. Evaporate  the  solutions  on  the  water-bath  until 
they  become  pasty ;  remove  the  dishes,  and,  when  nearly  cold, 
pour  over  the  crystals  about  25  cubic  centimetres  of  rectified 
methylated  spirit.  Do  not  attempt  to  break  up  the  highly 
crystalline  scales  of  the  double  salt  The  action  of  the 
alcohol  in  dissolving  out  the  excess  of  platinum  tetrachloride 
may  be  facilitated  by  imparting  a  gentle  rotatory  motion  to 
the  contents  of  the  dish.*  Cover,  and  allow  to  stand  for 
five  or  ten  minutes ;  pour  the  clear  liquid  on  to  a  No.  2 
filter  (which  ought  to  be  first  washed  with  hot  water,  and 
then  with  alcohol),  and  drain  the  liquid  as  far  as  possible 
from  the  precipitate  :  again  add  about  10  cubic  centimetres 
of  spirit  to  the  precipitate,  and  shake ;  allow  to  stand  five 
minutes,  and  pour  the  clear  liquid  through  the  filter. 
Repeat  the  digestion  with  spirit  a  third  and  fourth  time  :  the 
solution  will  now  be  nearly  colourless.  Transfer  the  precipi- 
tate to  a  weighed  crucible,  by  the  aid  of  a  glass  rod,  and  a 
stream  of  alcohol  from  a  small  wash-bottle  ;  pour  the  alcohol 
through  the  filter,  and  wash  the  paper  carefully  with  alcohol 
until  the  filtrate  is  absolutely  colourless.  Dry  the  double 
chloride  at  70°,  heat  to  100°  in  the  steam -chamber,  and 
weigh  the  precipitate.  If  the  above  instructions  have  been 
properly  followed,  scarcely  a  stain  will  be  left  on  the  paper. 
The  filter  is  burnt,  the  ash  added,  and  the  whole  again 
weighed  :  the  increase  in  the  weight  of  the  crucible  after 
subtracting  the  ash,  may  without  sensible  error  be  considered 
as  platinum  :  it  is  calculated  to  K2Ptd6,  and  the  amount 

*  Water  would  dissolve  the  double  chloride :  100  parts  of  water 
dissolve  about  I  part  of  the  salt  at  the  ordinary  temperature ;  whereas 
the  same  quantity  of  the  salt  requires  26,400  parts  of  alcohol  of  80  per 
cent,  and  42, 6co  parts  of  absolute  alcohol. 


Dolomite.  85 

added  to  the  main  quantity.  When  calculated  to  potassium 
carbonate,  the  result  of  the  two  determinations  ought  not 
to  differ  by  more  than  o-2  per  cent. 

IV.    ROCHELLE  SALT.     C4H4KNaO6 .  4H2O. 
(Separation  of  Potassium  and  Sodium.} 

Preparation. — 40  grams  of  cream  of  tartar,  and  30  grams 
of  crystallised  sodium  carbonate,  are  added  successively  in 
small  portions  at  a  time,  to  150  cubic  centimetres  of  boiling 
water.  The  liquid  must  be  tested  to  ascertain  that  it  is 
alkaline ;  it  is  filtered,  if  necessary,  and  concentrated  by 
evaporation.  On  cooling,  the  solution  deposits  fine  large 
crystals  of  the  potassium-sodium-tartrate. 

Analysis. — Powder  a  few  of  the  crystals,  press  between 
filter-paper,  and  weigh  off  from  a  tube  about  1*5  gram 
into  a  platinum  dish  or  crucible.  Heat  gently,  so  as  to 
melt  the  salt  in  its  water  of  crystallisation,  and  gradually 
increase  the  flame  until  the  mass  is  perfectly  dry.  Ignite  at 
a  low  red  heat  in  the  draught-chamber  for  some  time ;  allow 
the  charred  mass  to  cool,  and  digest  it  repeatedly  with  hot 
water;  filter,  acidulate  with  pure  hydrochloric  acid,  and 
evaporate  to  dryness  in  a  weighed  platinum  dish,  with  all 
the  precautions  necessary  to  avoid  loss  by  spirting.  As  soon 
as  the  residue  is  perfectly  dry,  heat  it  very  gently  for  five 
minutes  over  the  lamp,  transfer  to  the  desiccator,  and  when 
cold  weigh  the  mixed  chlorides.  Dissolve  in  a  small 
quantity  of  water,  transfer  to  a  porcelain  dish,  and  separate 
the  potassium  with  platinum  tetrachloride,  as  directed  in  the 
foregoing  example.  Enough  platinum  chloride  must  be 
added  to  convert  both  the  alkaline  chlorides  into  the 
double  salts  of  platinum.  The  sodium-platinum-chloride  is 
readily  soluble  in  alcohol,  especially  if  previously  moistened 
with  water. 

V.  DOLOMITE. 

This  substance  is  essentially  a  double  carbonate  of 
lime  and  magnesia.  No  definite  relation,  however,  exists 


86 


Quantitative  Chemical  Analysis. 


between  the  amounts  of  the  two  carbonates,  as  the  calcium 
and  magnesium  replace  each  other  in  all  proportions.  Occa- 
sionally the  mineral  occurs  associated  with  the  isomorphous 
carbonates  of  iron  and  manganese. 

The  portion  employed  for  analysis  is  finely  powdered,  dried 
in  the  steam-bath,  and  introduced  into  a  small  corked  tube. 

Determination  of  the  Carbonic  Acid. — The  mineral  is 
decomposed  by  dilute  hydrochloric  acid,  and  the  carbon 

FIG.  31. 


dioxide,  freed  from  moisture,  is  absorbed  by  soda-lime. 
The  determination  is  most  accurately  effected  by  means  of 
the  apparatus  represented  in  fig.  31.  The  flask  A  has  a 
capacity  of  about  150  cubic  centimetres  :  it  is  fitted  with  a 
caoutchouc  cork,  containing  two  holes,  into  one  of  which  is 
inserted  the  little  bulb-tube  a,  containing  a  few  drops  of 
strong  sulphuric  acid.  This  serves  to  regulate  the  rapidity 
of  the  decomposition,  by  indicating  the  speed  with  which 
the  gas  travels  through  the  apparatus.  The  tube  £,  which 


Dolomite.  87 

may  be  10  centimetres  high,  is  filled  to  a  depth  of  6  centi- 
metres with  pumice  saturated  with  solution  of  copper  sul- 
phate, and  strongly  heated  until  all  the  water  has  been 
expelled  :  its  use  is  to  absorb  any  vapour  of  hydrochloric 
acid  which  may  pass  over  from  the  flask.  The  remainder 
of  the  tube  is  filled  with  coarsely-powdered  calcium  chlo- 
ride. In  the  tube  c  the  carbonic  acid  is  absorbed  ;  seven- 
eighths  of  it  are  filled  with  soda-lime ;  and  as  this  sub- 
stance, in  combining  with  carbonic  acid,  becomes  heated, 
and  parts  with  a  little  water,  the  remaining  one-eighth  of 
the  tube  is  filled  with  calcium  chloride  :  c  is  also  con- 
nected with  a  small  unweighed  tube  containing  calcium 
chloride,  in  order  to  prevent  the  possibility  of  the  weighed 
tube  absorbing  atmospheric  moisture.  Caoutchouc  corks  are 
used  to  close  the  tubes ;  if,  in  their  absence,  ordinary  corks 
are  employed,  they  must  be  cut,  and  covered  with  sealing- 
wax.  The  tubes  may  be  suspended  as  in  the  figure,  or  by 
wires  from  a  glass  rod  running  through  a  cork  held  in  a 
clamp  on  the  retort  stand.  When  not  in  use  the  tubes  are 
closed  by  stoppers  of  glass  rod.  In  the  other  hole  of  the 
caoutchouc  stopper  of  A  is  fitted  a  bulb-tube,  passing  down 
nearly  to  the  bottom  of  the  flask,  drawn  out  and  turned 
up  in  the  manner  represented  in  the  figure  ;  this  can  be 
closed  by  means  of  a  small  screw  clamp  :  d  contains  soda- 
lime  :  it  is  attached  to  the  caoutchouc  tubing  on  the  bulb- 
tube  at  the  close  of  the  experiment. 

Weigh  out  about  1-5  gram  of  the  carbonate  into  A,  and 
add  about  i  o  cubic  centimetres  of  water.  Weigh  c  without 
the  stoppers  and  tubing,  and  put  the  several  parts  of  the 
apparatus  together,  setting  the  flask  A  upon  a  piece  of  wire 
gauze  on  a  tripod.  Nearly  fill  the  bulb-tube  with  hydro- 
chloric acid  solution,  diluted  with  its  own  volume  of  water. 
Open  the  clamp,  and  allow  enough  acid  to  pass  into  the 
flask  to  set  up  the  evolution  of  the  carbonic  acid,  and  as  the 
action  diminishes  add  more  acid,  until  the  tube  is  empty. 
Close  the  clamp,  and  connect  the  exit-end  of  the  apparatus 
with  an  inverted  wash-bottle,  of  about  600  cubic  centimetres 


88  Quantitative  Chemical  Analysis. 

capacity,  and  filled  with  water,  the  jet  of  which  you  have 
transferred  to  the  short,  obtusely-bent  tube,  and  temporarily 
closed  by  a  piece  of  caoutchouc  tubing  and  clamp;  heat 
A  slowly  to  boiling,  boil  for  about  ,one  minute,  cautiously 
open  the  clamps,  remove  the  lamp,  and  aspirate  a  slow 
current  of  air  through  the  apparatus,  regulating  the  speed  by 
means  of  the  screw.  Detach  c,  allow  it  to  cool,  and  weigh  it; 
the  increase  of  .weight  gives  the  amount  of  carbonic  acid  in 
the  mineral.  This  method  of  estimating  carbon  dioxide  is 
very  accurate,  and  is  generally  applicable  ;  it  is  expeditious, 
and  has  the  advantage  of  being  direct.  The  tubes  may  be 
used  a  great  number  of  times  without  their  contents  being 
changed,  if  they  are  well  stoppered  when  not  in  use. 
A.  very  simple  and  accurate  method  of  determining  carbon 
dioxide  in  salts  and  minerals  consists  in  heating  the  sub- 
stance with  fused  and  powdered  potassium  bichromate  in  a 
short  combustion-tube  (about  25  cm.  long),  passing  the 
evolved  gas  through  a  tube  containing  calcium  chloride,  and 
absorbing  it  in  a  weighed  soda-lime  tube.  Or  a  weighed 
portion  of  the  carbonate  is  heated  with  about  four  times 
the  quantity  of  fused  borax  in  a  platinum  crucible  to  dull 
redness,  and  the  carbon  dioxide  determined  from  the  loss 
of  weight. 

Determination  of  the  Silica,  Iron  (and  Manganese),  Lime, 
and  Magnesia. — Decant  the  solution  from  the  flask  A  into  a 
porcelain  basin,  rinse  the  flask  with  a  few  cubic  centimetres 
of  hot  water,  adding  the  washings  to  the  main  quantity  of  the 
liquid,  and  evaporate  the  whole  to  complete  dryness  on  the 
water-bath,  in  order  to  render  the  small  quantity  of  silica 
insoluble.  Moisten  the  dried  residue  with  a  few  drops  of 
strong  hydrochloric  acid,  cover  the  basin  with  a  glass  plate, 
allow  to  stand  for  a  few  minutes,  add  hot  water,  and  filter 
the  solution  through  a  No.  3  filter ;  wash  thoroughly,  drain 
the  paper  by  the  action  of  the  pump,  fold  the  filter,  and 
without  further  drying  throw  it  into  a  weighed  platinum 
crucible  :  cautiously  heat  until  the  paper  is  dry,  and  incine- 


Dolomite.  89 

rate.  The  increase  in  weight  of  the  crucible,  minus  the 
filter-ash,  gives  the  amount  of  silica. 

To  the  filtrate  from  the  silica,  add  a  little  bromine-water, 
then  ammonium  chloride  and  ammonia  in  slight  excess  ; 
heat  gently  for  some  time  and  filter.  Wash  the  precipitate 
once  or  twice,  re-dissolve  it  in  hydrochloric  acid  by  heating, 
add  one  more  drop  of  bromine,  again  precipitate  with  ammo- 
nia, and  filter.  Dry  and  weigh  the  oxide  of  iron  (and  man- 
ganese*): the  precipitate  may  be  ignited  without  complete 
drying.  The  second  precipitation  effects  the  removal  of  small 
quantities  of  lime  and  magnesia  precipitated  with  the  ferric 
oxide. 

Mix  the  ammoniacal  filtrates,  and  add  ammonium  oxalate 
in  quantity  sufficient  to  precipitate  the  lime  and  to  convert 
the  magnesia  into  oxalate.  Presence  of  excess  of  ammonium 
oxalate  prevents  the  slight  solubility  of  calcium  oxalate  in 
chloride  of  ammonium  solution.  Allow  the  liquid  to  stand 
for  ten  or  twelve  hours.  Decant  the  clear  liquid  on  to  a  filter 
and  wash  once  or  twice  by  decantation,  taking  care  to  disturb 
the  precipitate  as  little  as  possible.  Dissolve  the  calcium 
oxalate  in  a  small  quantity  of  hydrochloric  acid,  heat  to 
boiling,  add  a  few  drops  of  ammonium  oxalate  and  a  slight 
excess  of  ammonia.  This  double  precipitation  of  the  calcium 
oxalate  effects  the  separation  of  a  small  quantity  of  co-pre- 
cipitated magnesia.  Filter  off  the  oxalate,  wash  thoroughly 
with  hot  water,  and  dry.  Transfer  the  dried  precipitate  to  a 
weighed  platinum  crucible,  and  if  its  weight  does  not  exceed 
i  gram,  proceed  to  convert  it  into  caustic  lime.  Burn  the 
filter,  and  add  the  ash  to  the  crucible.  Heat  the  crucible 
gently,  with  the  lid  on,  and  gradually  increase  the  flame 
until  the  bottom  is  red  hot.  Now  expose  the  crucible  to  a  full 
red  heat  over  the  blow-pipe  for  fifteen  minutes,  occasionally 
removing  the  lid  for  a  few  seconds  ;  allow  to  cool  in  the 
desiccator,  and  weigh.  By  this  treatment  the  oxalate  is  con- 
verted, first  into  carbonate  and  then  into  caustic  lime.  The 

*  For  a.  method  of  determining  the  manganese,  usually  present  in 
small  quantity  only,  in  limestones  and  dolomites,  see  Part  IV. 


90  Quantitative  Chemical  Analysis. 

heating  must  be  repeated  until  the  weight  is  perfectly  con* 
stant ;  that  is,  until  the  carbonate  is  wholly  converted  into 
lime.  If  the  quantity  of  the  oxalate  exceeds  i  gram,  its 
conversion  into  oxide  is  accomplished  with  difficulty  and 
requires  prolonged  heating.  In  this  case  it  is  better  to  trans- 
form the  oxalate  simply  into  carbonate  by  heating  gently  over 
a  small  flame  scarcely  sufficient  to  make  the  bottom  of  the 
crucible  appear  red  hot  by  diffuse  daylight.  The  conversion 
into  carbonate  is  rendered  visible  by  a  slight  change  of  colour 
which  creeps  over  the  heated  oxalate.  After  heating  for  ten 
minutes,  weigh,  and  repeat  the  operation  with  the  lid  on 
until  the  weight  is  constant.  Weigh  the  filter-ash  and  the 
small  quantity  of  adhering  lime  on  the  lid.  Moisten  the 
carbonate  with  a  few  drops  of  water  and  test  it  with  a  slip  of 
reddened  litmus  paper  ;  it  ought  not  to  show  the  slightest 
trace  of  alkalinity.  If  the  paper  becomes  blue,  the  crucible 
has  been  overheated  ;  in  that  case  transfer  the  ash  of  the 
filter  to  the  crucible,  add  a  few  drops  of  ammonium  carbonate 
solution,  evaporate  to  dryness  on  the  water-bath,  heat  very 
gently  for  a  few  minutes  and  weigh.  The  ammonium  car- 
bonate reconverts  the  caustic  lime  into  carbonate.  The 
nitrate  from  the  oxalate  of  lime  is  poured  into  a  porcelain 
basin,  concentrated  considerably,  and  rendered  strongly  acid 
by  nitric  acid.  About  3  grams  of  the  acid  are  employed  for 
each  gram  of  the  sal-  ammoniac  supposed  to  be  present.  The 
dish  is  covered  with  an  inverted  funnel  and  gently  heated, 
when  a  rapid  effervescence  sets  in,  owing  to  the  decompo- 
sition of  the  ammonium  chloride.  The  liquid  is  evaporated 
to  dryness,  when,  if  sufficient  nitric  acid  has  been  added,  all 
the  sal-ammoniac  will  have  been  expelled.*  The  funnel  is 
rinsed,  and  the  saline  mass  in  the  dish  dissolved  in  water, 
ammonium  chloride  added  in  small  quantity,  the  solution 
rendered  strongly  alkaline  by  ammonia,  filtered  if  necessary, 
and  mixed  with  solution  of  sodium  phosphate.  The  liquid 

*  This  method  of  removing  the  excess  of  sal-ammoniac  is  preferable 
to  that  of  evaporating  to  dryness  in  a  platinum  basin  and  igniting.  By 
the  latter  plan  there  is  danger  of  loss  from  the  tendency  of  the  salt  to 
creep  over  the  side  of  the  dish  during  the  evaporation. 


Barium  Sulphate.  91 

is  well  agitated  by  shaking  the  beaker,  covered  with  a  glass 
plate,  and  set  aside  for  twenty-four  hours.  The  clear  liquid 
is  poured  on  to  a  filter  and  the  precipitate  washed  by  decan- 
tation  in  the  beaker,  and  afterwards  on  the  filter  by  dilute 
ammonia-water  (i  part  strong  ammonia  and  5  of  water), 
until  the  filtrate  acidulated  with  pure  nitric  acid  gives  only 
a  slight  opalescence  with  silver  nitrate.  Pure  water  dissolves 
the  precipitate  to  a  slight  extent  (i  part  in  15,300  of  water); 
in  dilute  ammonia  it  is  much  less  soluble  (i  part  in  45,000). 
The  presence  of  a  large  excess  of  ammonium  chloride  in- 
creases its  solubility;  hence  the  necessity  of  expelling  the 
greater  portion  of  this  salt  before  precipitating  the  magnesia. 
The  dried  phosphate  is  detached  as  completely  as  possible 
from  the  filter,  transferred  to  a  weighed  platinum  crucible  and 
very  gradually  heated  (at  first  with  the  lid  on)  for  fifteen 
minutes.  The  temperature  is  now  raised  until  the  crucible 
is  red  hot,  when  the  lamp  is  withdrawn  and  the  filter-ash 
added.  Take  care  that  the  filter  is  burnt  as  completely 
as  possible,  or  the  crucible  will  be  corroded  by  the  action  of 
the  carbon  on  the  phosphate.  The  crucible  is  then  strongly 
ignited  for  a  quarter  of  an  hour,  allowed  to  cool,  and  weighed. 
The  residue  (magnesium  pyrophosphate  Mg2  P2O7)  should 
be  white,  or,  at  most,  have  a  very  slight  tinge  of  grey.* 

VI.     BARIUM  SULPHATE  (Ba  SO4). 

The  pure  substance  is  prepared  by  adding  a  dilute  solu- 
tion of  barium  chloride  to  an  excess  of  hot  and  moderately 
diluted  sulphuric  acid.  The  precipitate  is  washed  once  or 
twice  with  hot  water,  dried,  and  ignited.  The  method  of 
analysis  is  founded  upon  the  decomposition  of  the  sulphate 
by  prolonged  fusion  with  an  alkaline  carbonate. 

About  i  gram  of  the  ignited  precipitate  is  weighed  out 

*  In  any  analysis  involving  the  separation  of  several  substances  it  is 
advisable  to  preserve  the  weighed  precipitates  in  small  corked  tubes  or 
between  watch-glasses  until  the  analysis  is  finished.  Questions  re- 
garding the  purity  or  identity  of  the  substances  separated  frequently 
arise,  which,  of  course,  cannot  be  answered  if  the  bodies  are  thrown 
away. 


92  Quantitative  Chemical  Analysis. 

into  a  platinum  crucible  and  mixed  with  3  parts  of  a  dry 
mixture,  in  equivalent  proportions,  of  pure  potassium  and 
sodium  carbonates.  This  mixture  may  be  conveniently 
obtained  in  a  state  of  purity  by  igniting  Rochelle  salt 
(C4H4KNaO6  +  4H2O)  in  a  platinum  basin,  extracting  the 
charred  residue  with  hot  water,  filtering  and  evaporating  to  dry- 
ness.  The  mixture  of  barium  sulphate  and  alkaline  carbonates 
is  fused  at  a  bright  red  heat  for  thirty  or  forty  minutes,  allowed 
to  cool,  the  mass  extracted  with  water  containing  a  few  drops 
of  ammonia  and  ammonium  carbonate,  and  filtered.*  The 
filtrate  contains  the  sulphuric  acid  in  union  with  the  alkalies; 
the  residue  consists  of  barium  carbonate.  It  is  washed  with 
water  containing  a  few  drops  of  ammonia  and  ammonium 
carbonate,  dried,  and  'weighed.  It  is  dissolved  in  the 
crucible  in  a  few  drops  of  hydrochloric  acid,  and  a  slight 
excess  of  sulphuric  acid  added;  the  mixture  is  cautiously 
evaporated  to  dryness,  and  the  residue  ignited  and  weighed. 
Its  amount  ought  to  be  equal  to  that  originally  taken. 

The  filtrate  containing  the  alkaline  sulphate  is  acidulated 
with  hydrochloric  acid,  heated  to  boiling,  barium  chloride 
added,  and  the  precipitate  washed,  dried,  ignited,  and 
weighed.  Its  weight  ought  to  equal  that  of  the  barium  sul- 
phate analysed. 

The  same  process  is  applicable  to  the  analysis  of  strontium 
and  calcium  sulphates. 

VII.    INDIRECT  ESTIMATION  OF  BARIUM  AND  CALCIUM. 

It  is  frequently  possible  to  determine  the  amount  of  a 
substance  A,  by  combining  it  with  a  second  body  B,  so  as  to 
form  a  definite  compound  A  B.  By  estimating  the  quantity 
of  B  in  the  combination,  we  can  readily  calculate  the  amount 
of  A  which  must  be  present.  Thus  we  can  determine  the 
amount  of  Ag  in  a  solution  by  estimating  the  amount  of  Cl 
required  to  precipitate  it  completely.  In  like  manner  we 

*  Barium  carbonate  dissolves  in  14,000  parts  of  cold  water  and 
15,500  of  boiling  water.  It  is  ten  times  less  soluble  in  water  con- 
taining a  slight  quantity  of  ammonia  and  ammonium  carbonate. 


Indirect  A  nalysis.  93 

could  determine  the  amount  of  Ba  and  Ca  from  the  quantity 
of  CO2  respectively  contained  in  their  carbonates.  Such 
indirect  determinations  are  based  on  the  law  of  constant 
proportion,  which  states  \h&\.  the  same  substance  always  consists 
of  the  same  elements  united  in  the  same  proportion. 

But  it  will  be  obvious  on  a  little  reflection  that  we  can 
determine  the  amount  of  barium  and  calcium  in  a  mixture  of 
their  carbonates,  by  estimating  the  amount  of  carbon  dioxide 
contained  in  a  known  weight  of  the  mixed  compounds.  The 
possibility  of  such  an  estimation  is  based  upon  the  wide 
difference  which  exists  between  the  combining  weight  of 
barium  (137*2)  and  that  of  calcium  (40-0),  both  of  which 
substances  combine  with  44  of  CO2.  Supposing  that  we  had 
found  that  2  grams  of  the  mixed  carbonates  had  evolved  0*67 
gram  of  carbon  dioxide.  Then,  if  the  whole  of  the  carbon 
dioxide  were  combined  with  the  barium,  the  amount  of  the 
barium  carbonate  would  be  3*002  grams. 

Eq.  C03  Eq.  BaCO3.  CO2  found. 

44         :  197-2       ::       0-67      :      =    3-002. 

But  the  weight  of  the  mixed  carbonates  taken  was  only  2 
grams.  The  deficiency  (3-002  —  2 -o)=  i  '002,  is  proportional 
to  the  amount  of  calcium  carbonate  present.  This  amount 
is  thus  found  :  The  difference  between  the  atomic  weights  of 
BaCO3  and  CaCO3  is  to  the  atomic  weight  of  CaCO3,  as  the 
difference  found  is  to  the  quantity  of  calcium  carbonate  contained 
in  the  mixture. 

BaCO3  -  CaCO3  =  197-2   -   100  =  97-2 

BaCO3  -  CaCO3.  CaCO3. 

97*2          :          100     ::     1*002      :       =     1-032. 
Accordingly  the  composition  of  the  mixture  is 

Calcium  Carbonate     1*032 
Barium  Carbonate      0*968 
2-000 
or  expressed  centesimally, 

Calcium  Carbonate      51-6 
Barium  Carbonate        48*4 

lOO'O 


94  Quantitative  Chemical  Analysis. 

Similarly  we  might  determine  the  proportion  of  the  bases 
present  in  the  mixture  by  estimating  the  weight  of  sulphuric 
acid  necessary  to  form  the  two  sulphates.  The  method  of 
calculation  is,  mutatis  mutandis,  precisely  similar  to  that 
above  given. 

Weigh  out  into  a  platinum  crucible  about  equal  weights 
of  pure  and  recently  ignited  barium  and  calcium  sulphates 
(o'5  gram  of  each  is  a  convenient  quantity  to  take).  Mix 
with  3  or  4  pts.  of  the  mixture  of  sodium  and  potassium 
carbonates,  and  proceed  exactly  as  described  in  the  preceding 
example.  The  weighed  barium  and  calcium  carbonates  are 
then  decomposed  in  the  apparatus  described  in  p.  84,  and 
the  amount  of  carbon  dioxide  determined  with  great  care. 
From  the  weight  of  CO2  obtained,  the  proportion  of  the  two 
bases  is  calculated  in  the  manner  above  described. 

As  a  control,  determine  the  amount  of  sulphuric  acid  in 
the  nitrate,  after  acidulation  with  hydrochloric  acid,  by  pre- 
cipitation with  barium  chloride,  according  to  the  method 
described  on  p-  75,  and  again  calculate  the  proportion  of  the 
two  bases.  This  exercise  will  afford  a  good  trial  of  the 
manipulative  skill  of  the  operator. 

VIII.    FERROUS  AMMONIUM  SULPHATE. 
Fe(NH4)2  2SO4  +  6H2O. 

To  prepare  this  salt,  27-8  grams  of  recrystallised  ferrous 
sulphate  and  13*2  grams  of  pure  ammonium  sulphate  are 
separately  dissolved  in  the  least  possible  quantity  of  water 
at  a  temperature  of  about  40°.  The  solutions  are  mixed,  a 
few  drops  of  sulphuric  acid  are  added,  and  the  mixture  is 
stirred  constantly  until  cold.  The  greater  portion  of  the 
salt  separates  out  in  a  finely-divided  state  :  if  the  solution  is 
now  set  aside  for  a  few  hours  a  further  quantity  of  the  double 
salt  crystallises  out.  Pour  off  the  mother-liquor,  allow  the 
crystalline  powder  to  drain,  and  dry  it  thoroughly  between 
filter-paper. 

Determination  of  the  Ammonia. — Weigh  out  about  i  gram 


Ferrous  Ammonium  Sulphate. 


95 


of  the  salt  into  the  retort  (fig.  32),  the  neck  of  which  is 
contracted  at  a,  and  the  upper  portion  filled  with  fragments 
of  broken  glass.  The  tube  b  is  filled  with  strong  soda-lye, 
which  can  be  delivered  little  by  little  on  opening  the  clamp. 
The  flask  c  is  fitted  with  a  caoutchouc  cork  and  bent  tube  <?, 
on  which  is  blown  a  bulb.  The  short  wide  tube  d  is  filled 
with  fragments  of  glass,  previously  well  washed  with  water  j 
through  this  tube  hydrochloric  acid  is  poured.  The  tube 
e  is  so  arranged  that  it  just  dips  beneath  the  surface  of 

FIG.  32. 


the  liquid  in  the  flask.  The  retort  containing  the  weighed 
quantity  of  ammoniacal  salt,  dissolved  in  a  small  quantity  of 
water,  is  fixed  on  a  clamp,  the  tube  e  inserted  into  its  neck, 
a  small  quantity  of  soda  solution  allowed  to  flow  into  the 
retort,  and  the  liquid  heated  to  boiling.  Care  must  be 
tiken  to  prevent  the  liquid,  if  it  shows  any  tendency  to 
froth,  from  passing  over  into  the  flask.  The  broken  glass 
in  the  neck  of  the  retort  tends  to  prevent  such  a  mishap. 
The  liquid  should  be  boiled  for  fifteen  or  twenty  minutes 


96  Quantitative  Chemical  Analysis. 

after  the  caustic  soda  solution  has  been  added.  When  the 
evolution  of  ammonia  is  finished,  the  tube  e  is  disconnected, 
the  powdered  glass  in  d  washed  with  distilled  water,  the 
tube  e  drawn  up  from  the  liquid  and  washed  with  distilled 
water.  The  ammoniacal  solution  is  poured  into  a  porcelain 
basin,  an  excess  of  platinum  tetrachloride  is  added,  and  the 
whole  is  evaporated  just  to  dryness  on  the  water-bath.  The 
double  chloride  is  washed  with  strong  alcohol,  and  is  trans- 
ferred to  the  weighed  platinum  crucible  in  the  manner  de- 
scribed on  p.  82.  The  salt  is  dried  at  70°  or  80°,  and  heated 
to  1 00°  for  ten  or  fifteen  minutes,  after  which  its  weight  will  be 
constant.  The  little  filter  is  dried  and  ignited  on  the  lid  of 
the  crucible  and  weighed  separately.  The  weight  of  the 
residual  platinum  is  calculated  to  that  of  double  salt,  and  the 
amount  added  to  the  main  quantity.  By  way  of  control,  the 
double  salt  may  be  gently  heated  so  as  to  expel  the  greater 
portion  of  the  ammonium  chloride ;  the  crucible  is  then 
raised  to  a  full  red  heat,  and  the  metallic  platinum  weighed. 
Determining  as  platinum,  however,  is  not  more  accurate  than 
weighing  the  double  salt,  owing  to  the  readiness  with  which 
finely-divided  particles  of  the  metal  are  carried  away  in  the 
vapour  of  the  escaping  ammonium  chloride. 

IX.    DETERMINATION  OF  NITRIC  ACID. 

Dr.  Gladstone  and  Mr.  Tribe  have  found  that  a  thin 
plate  of  zinc  coated  with  copper  (formed  by  placing  the 
former  metal  in  a  solution  of  copper  sulphate  for  a  few 
minutes)  decomposes  water,  particularly  on  warming,  with 
the  formation  of  zinc  hydrate  and  the  evolution  of  hydrogen. 

The  hydrogen  so  eliminated  is  capable  of  reducing  nitric 
acid  in  combination  to  the  state  of  ammonia : 

NO3K  +  4H2     =     NH3  +   HKO   +   2H2O. 

This  reaction  constitutes  the  basis  of  a  method  of  deter- 
mining nitric  acid  in  nitrates. 

About  25-30  grams  of  thin  sheet  zinc  are  placed  in  a  flask 


Nitric  Acid.  97 

of  about  200  c.c.  capacity,  and  covered  with  a  moderately- 
concentrated  and  slightly-warmed  solution  of  copper  sul- 
phate. In  about  ten  minutes  a  thick  spongy  coating  of 
copper  will  be  deposited  on  the  zinc  :  the  liquid  is  poured 
off  the  metals,  which  are  now  well  washed  with  cold  water, 
and  covered  with  about  40  or  50  c.c.  of  pure  water.  Weigh 
out  about  0-5  gram  of  pure  nitre  into  the  flask,  which  is  then 

FIG.  33. 


placed  in  a  sand-bath  and  connected  with  the  condensing 
arrangement  shown  in  fig.  33.  The  receiver  and  U  tube 
contain  a  few  cubic  centimetres  of  dilute  hydrochloric  acid. 
The  liquid  is  gradually  heated  to  boiling  and  distilled  for 
about  an  hour.  The  distillate  is  poured  from  the  receiver 
and  evaporated  to  dryness  in  a  porcelain  basin  over  the 
water-bath,  with  excess  of  platinum  tetrachloride,  and  the 
double  chloride  is  treated  exactly  as  in  the  foregoing  example. 


98  Quantitative  Chemical  Analysis. 


X. .   POTASH-ALUM.     A12(SO4)3.K2SO4.24H2O. 

The  salt  is  purified  by  recrystallisation,  powdered,  dried 
between  blotting-paper,  and  placed  in  a  well-corked  tube. 

Determination  of  the  •  water. — About  i  gram  of  the 
double  salt  is  heated  to  120°  in  the  apparatus  represented  in 
fig.  29,  until  it  ceases  to  lose  weight.  The  loss  is  equiva- 
lent to  10  atoms  of  water.  The  temperature  is  now  raised 
to  200°,  and  the  heat  maintained  at  this  point  until  the 
weight  is  once  more  constant.  The  salt  should  now  be 
perfectly  anhydrous. 

Determination  of  the  Alumina  and  Potassium  Sulphate. — 
Weigh  out  about  i  gram  of  the  crystalline  salt  into  a  porce- 
lain basin,  dissolve  in  hot  water,  and  add  a  moderate 
quantity  of  ammonium  chloride  solution,  together  with  a 
slight  excess  of  ammonia.  Heat  the  liquid  to  boiling  and 
maintain  it  in  gentle  ebullition  for  some  time,  keeping  the 
basin  covered  with  a  sufficiently  large  watch-glass  to  avoid 
loss  by  projection.  Rinse  the  watch-glass  into  the  basin, 
and  pour  the  clear  supernatant  liquid  on  to  the  filter,  wash 
the  precipitate  once  or  twice  by  decantation  and  transfer  it 
also  to  the  filter  ;  wash  three  or  four  times  with  boiling  water, 
and,  after  the  whole  of  the  liquid  has  passed  through,  keep 
up  the  action  of  the  pump  for  ten  minutes.  Remove  the 
filter  containing  the  precipitate  from  the  funnel,  and,  without 
further  drying,  place  it  in  a  weighed  platinum  crucible ;  heat 
gently  for  a  few  minutes  to  char  the  paper,  and  gradually  in- 
crease the  flame  until  the  crucible  is  red  hot.  Keep  it  at  this 
temperature  for  ten  or  fifteen  minutes,  occasionally  removing 
the  lid,  and  then  ignite  it  strongly  over  the  blow-pipe  for 
five  or  ten  minutes ;  place  the  crucible  in  the  desiccator,  and 
weigh  when  cold. 

Ignition  over  the  blast-lamp  expels  the  last  traces  of  water, 
together  with  the  minute  quantity  of  sulphuric  acid  which 


Glass.  99 

is  precipitated  with  the  alumina  from  a  solution  containing 
sulphates. 

The  filtrate  from  the  precipitate  of  alumina  contains  the 
potassium  sulphate  :  it  is  evaporated  to  dryness  in  a  weighed 
platinum  basin,  gently  heated  to  expel  ammoniacal  salts,  and 
moistened  with  a  few  drops  of  pure  sulphuric  acid^  in  order 
to  transform  the  potassium  chloride,  which  invariably  forms 
when  potassium  sulphate  is  heated  with  ammonium  chloride, 
back  into  sulphate.  The  mass  is  again  heated,  with  all  the 
precautions  detailed  on  p.  82,  in  order  to  expel  the  excess 
of  sulphuric  acid,  and  when  cold,  the  potassium  sulphate 
is  weighed. 

XI.     GLASS 

consists  of  a  mixture  of  the  alkaline  silicate  with  cer- 
tain insoluble  silicates,  generally  of  calcium,  lead,  iron, 
aluminium,  magnesium,  or  manganese.  The  best  window 
glass  has  approximately  the  composition,  Na2OQxO.6SiO2. 
In  flint  glass  the  lime  is  replaced  by  oxide  of  lead.  The 
pale  green  variety  used  for  chemical  apparatus  is  mainly 
made  up  of  silicates  of  lime  and  potash,  mixed  with  smaller 
quantities  of  iron  and  alumina. 

In  order  to  analyse  it,  the  glass  is  reduced  to  the  finest 
possible  state  of  division,  and  fused  in  a  platinum  crucible 
with  four  times  its  weight  of  a  mixture  of  equal  parts  of 
sodium  and  potassium  carbonates.  When  cold,  the  crucible 
is  placed  in  a  porcelain  basin,  and  the  mass  boiled  out  with 
water,  hydrochloric  acid  is  added  in  excess,  and  the  whole 
is  evaporated  to  complete  dryness  over  the  water-bath.  The 
dried  mass  is  then  moistened  with  strong  hydrochloric  acid, 
hot  water  is  added,  and  the  silica  is  filtered  off,  repeatedly 
washed  with  hot  water,  dried  and  weighed.  The  solution  con- 
tains the  lead,  iron,  alumina,  manganese,  lime,  and  magnesia. 
The  alkalies  cannot,  of  course,  be  determined  in  this  por- 
tion, as  they  are  mixed  with  the  salts  required  to  decompose 

II  2 


IOO  Quantitative  Chemical  Analysis. 

the  glass.  Pass  sulphuretted  hydrogen  through  the  filtrate, 
to  precipitate  the  lead ;  filter,  dry  it,  and  convert  it  into 
sulphate  by  treatment  with  strong  nitric  acid.  Add  a  few 
drops  of  bromine  to  the  filtrate,  and  heat  gently  ;  add 
ammonia,  and  filter  off  the  iron,  alumina,  and  manganese. 
The  lime  and  magnesia  are  separated  as  in  No.  V. 


FIG.  34.  FIG.  35. 


Determination  of  the  Alkalies. — About  1*5  gram  of  the 
finely-powdered  glass  is  weighed  out  into  a  platinum  crucible, 
and  intimately  mixed  with  9  grams  of  calcium  carbonate,  and 
i  *5  gram  of  ammonium  chloride,  and  heated  to  bright  red- 
ness for  an  hour,  in  a  small  table  furnace  (figs.  34,  35).  The 
platinum  crucible  should  be  protected  from  the  direct  action 
of  the  fire  by  being  placed  in  a  small  clay  crucible,  with  a 


Glass.  IOI 

little  calcined  magnesia  at  the  bottom.  When' cold  the 
contents  of  the  crucible  are  boiled  with  water  in  a  silver  or 
platinum  dish,  filtered,  the  filtrate  acidified  with  hydrochloric 
acid,  and  evaporated  to  dryness  to  render  the  silica  in- 
soluble. The  residue  is  treated  with  hot  water  and  filtered ; 
to  the  filtrate,  ammonia,  ammonium  carbonate,  and  a  few 
drops  of  ammonium  oxalate  are  added  to  throw  down  the 
lime.  The  liquid  is  boiled,  to  render  the  precipitate  dense 
and  granular.  It  is  filtered  off ;  the  liquid  is  evaporated  to 
a  small  bulk  in  a  porcelain  basin,  pure  nitric  acid  is  added 
in  quantity,  and  the  whole  is  evaporated  to  dryness  to  de- 
stroy the  ammonium  chloride.  The  saline  residue  is  dis- 
solved in  a  little  water,  and  filtered  if  not  quite  clear,  and 
again  evaporated  to  dryness  with  a  small  quantity  of  strong 
hydrochloric  acid,  whereby  the  nitric  acid  is  expelled.  If 
the  quantity  of  the  mixed  alkalies  is  considerable,  this 
treatment  with  hydrochloric  acid  must  be  repeated  once  or 
twice  before  the  nitric  acid  is  completely  dissipated.  The 
alkaline  chlorides  are  again  dissolved  in  a  little  water,  and 
evaporated  to  dryness  in  a  weighed  platinum  dish,  heated 
gently,  and  weighed.  The  potassium  chloride  is  then  sepa- 
rated by  platinum  tetrachloride  in  the  manner  described  in 
No.  IV.  p.  83.  Its  amount,  subtracted  from  the  sum  of  the 
chlorides,  gives  the  sodium  chloride. 


XII.     FELSPAR  (Orthoclase,  Albite}. 

The  group  of  the  felspars  may  be  regarded  as  silicates  of 
alumina  united,  in  varying  proportions,  with  silicates  of  the 
alkalies  and  alkaline  earths.  The  varieties,  orthoclase  and 
albite,  differ  from  one  another  in  crystalline  form  and  in 
chemical  composition.  Orthoclase  crystallises  in  forms 
belonging  to  the  monoclinic  system,  and  the  alkali  it  contains 
is  chiefly  potash,  whereas  albite  is  triclinic,  and  its  alkali 
consists  mainly  of  soda. 


102       t   .  jQiiQ_ntita.tive  Chemical  A  nalysis. 

The  following  analyses  serve  to  show  this  characteristic 
difference  in  composition  : — 

Orthoclase  Albite 

Silica  6476  67-62 


Alumina 

Ferric  oxide 

Lime 

Magnesia 

Potash 

Soda. 

Loss  on  ignition 


17-60  16-59 

0-50  2-30 

0-65  0-85 

0-30  1-46 

I4'i8  0-51 

175  10-24 
0-65 

100-39  99-57 


The  methods  employed  in  the  analysis  of  these  minerals 
are  identical  with  those  described  in  No.  XL  The  only 
point  which  needs  special  mention  is  the  separation  of  the 
iron  and  alumina. 

The  solution  containing  the  iron,  alumina,  lime,  magnesia, 
and  alkalies,  from  which  the  silica  has  been  removed  by 
evaporation,  is  mixed  with  a  little  nitric  acid,  boiled  for  some 
time,  and  a  slight  excess  of  ammonia  added,  whereby,  on 
boiling,  the  iron  and  alumina  are  precipitated.  Care  must  be 
taken  not  to  employ  a  very  large  excess  of  ammonia,  other- 
wise the  precipitation,  even  after  protracted  boiling,  'will  not 
be  complete.  The  mixed  oxides  are  washed -thoroughly  with 
hot  water,  dried  as  far  as  possible  by  the  action  of  the  pump, 
and  transferred  to  a  platinum  dish  ;  the  small  portions  remain- 
ing on  the  filter  are  dissolved  in  hot  hydrochloric  acid,  the 
solution  being  allowed  to  drop  into  the  dish.  When  the  whole 
of  the  acid  solution  has  passed  through,  the  dish  is  removed 
from  under  the  funnel,  a  beaker  put  in  its  place,  and  the 
filter  thoroughly  washed  with  hot  water,  the  washings  being 
collected  in  the  beaker.  The  precipitate  in  the  dish  is  now 
dissolved  in  the  least  possible  quantity  of  hydrochloric  acid, 
an  excess  of  a  concentrated  solution  of  pure  caustic  potash 
added,  the  liquid  heated  to  boiling,  and  a  lump  of  the  pure 
hydrate  added,  in  quantity  sufficient  to  dissolve  the  alumina, 
and  the  boiling  is  continued  for  a  few  minutes.  The  contents 
of  the  dish  are  now  washed  into  the  beaker,  diluted  with  a  little 


Brass,  Bronze,  &c.  103 

water,  and  filtered,  the  ferric  oxide  being  repeatedly  washed 
with  hot  water,  dried,  and  weighed.  The  alkaline  solution 
containing  the  alumina  is  acidified  with  hydrochloric  acid, 
a  few  crystals  of  potassium  chlorate  are  added  to  destroy  any 
organic  matter  present,  which  would  tend  to  retain  a  small 
portion  of  the  alumina  in  solution ;  the  liquid  is  concentrated 
in  a  porcelain  basin,  and  ammonia  is  added  in  slight  excess, 
and  the  liquid  is  boiled  until  a  piece  of  turmeric  paper  held 
in  the  steam  is  no  longer  turned  brown.  The  precipitate  is 
filtered,  dried,  ignited  over  the  blow-pipe  in  a  platinum 
crucible,  at  first  very  gently,  and  with  the  lid  on,  and  then 
for  5  or  10  minutes  to  bright  redness. 

Instead  of  treating  the  mixed  oxides  of  iron  and  alumina 
with  caustic  potash,  they  may  be  washed,  dried,  ignited 
over  the  blow-pipe,  and  weighed  together.  The  mixture,  or 
an  aliquot  portion  of  it,  is  then  brought  into  a  porcelain 
boat,  and  strongly  heated  in  a  porcelain  tube,  in  a  current 
of  dry  hydrogen  for  an  hour.  The  boat  must  be  allowed  to 
cool  in  the  current  of  the  hydrogen  before  it  is  withdrawn. 
The  loss  of  weight  which  it  suffers  gives  the  amount  of 
oxygen  combined  with  the  iron ;  each  milligram  of  loss  is 
equivalent  to  3*339  milligrams  of  ferric  oxide.  With  proper 
care  this  method  is  very  accurate.  By  way  of  control 
(and  this  is  more  particularly  necessary  when  the  amount 
of  oxide  of  iron,  compared  with  that  of  the  alumina,  is  very 
small),  you  may  treat  the  weighed  mixture  with  highly 
dilute  nitric  acid  (i  part  of  acid  to  30  of  water).  The  dis- 
solved iron  is  re-precipitated,  after  filtering,  by  means  of 
ammonia,  dried,  and  weighed.  The  residual  alumina  is 
also  dried  and  weighed. 

XIII.  BRASS,  BRONZE,  GUN-METAL,  BELL-METAL. 
(Separation  of  Tin,  Copper,  Lead,  Iron,  and  Zinc.) 

Weigh  out  about  5  grams  of  the  finely-divided  alloy  (a 
portion  of  a  penny,  for  example)  into  a  flask,  and  dissolve  in 


IO4  Quantitative  Chemical  Anal} 


'StS. 


25  c.c.  strong  nitric  acid  and  15  c.c.  water  at  a  gentle  heat. 
Place  a  small  funnel  in  the  neck  of  the  flask  to  prevent  loss 
by  spirting.  When  the  substance  is  dissolved,  add  about 
three^  times  the  bulk  of  water,  and  digest  the  precipitate  on 
the  water  bath  with  occasional  shaking  for  about  an  hour. 
Allow  the  precipitate  to  settle,  decant  the  clear  liquid,  and 
repeat  the  digestion  on  the  water  bath  for  about  an  hour 
with  dilute  nitric  acid  (i  :  6).  The  oxide  of  tin  is  thus 
obtained  free  from  admixed  metals ;  it  is  filtered  off, 
washed,  dried,  and  weighed  in  a  porcelain  crucible.  If  the 
quantity  is  at  all  considerable,  it  requires  to  be  ignited  over 
the  blow-pipe  before  it  is  rendered  completely  anhydrous. 
The  filtrate  is  evaporated  nearly  to  dryness  with  strong  hydro- 
chloric acid,  to  expel  the  greater  portion  of  the  nitric  acid  ; 
re-dissolved  in  hot  water,  and  the  solution  precipitated  by 
sulphuretted  hydrogen.  The  clear  liquid  (which  should  smell 
strongly  of  the  gas)  is  filtered  off,  and  the  precipitate  washed 
once  or  twice  by  decantation  with  hydrochloric  acid  of 
sp.  gr.  i -05,  through  which  a  stream  of  sulphuretted  hydrogen 
is  led,  and  afterwards  with  water  containing  sulphuretted 
hydrogen.  The  mixed  sulphides  are  drained  thoroughly, 
and  transferred  to  a  small  porcelain  basin,  and  digested  with 
nitric  acid  and  about  10  cubic  centimetres  of  dilute  sul- 
phuric acid.  The  solution  is  evaporated  nearly  to  dryness, 
a  small  quantity  of  water  is  added  to  dissolve  the  copper 
sulphate,  and  the  lead  sulphate  is  filtered  off  without  delay 
through  as  small  a  filter  as  possible,  and  washed  with 
water  acidulated  with  sulphuric  acid,  the  filtrate  being  re- 
ceived in  a  litre  flask.  When  the  copper  has  been  washed 
away,  the  lead  sulphate  is  washed  with  dilute  alcohol  to 
remove  the  last  traces  of  acid,  otherwise  the  filter-paper 
would  blacken  on  drying  and  fall  to  pieces.*  The  sulphate 
is  weighed  in  a  porcelain  crucible ;  care  must  be  taken-  to 
remove  the  precipitate  as  completely  as  possible  from  the 

*  The  alcoholic  washings  are  not  to  be  mixed  with  the  filtrate  con- 
taining the  copper. 


Brass,  Bronze,  &c.  105 

filter  before  incinerating.  The  ash  may  be  moistened  with 
one  drop  of  dilute  nitric  acid,  heated  gently,  a  drop  of 
sulphuric  acid  added,  and  the  contents  of  the  crucible  care- 
fully dried  and  ignited.  The  filtrate  in  the  litre  flask  con- 
taining the  copper  is  diluted  to  the  mark,  and  the  liquid 
thoroughly  mixed  by  shaking;  100  cubic  centimetres  are 
withdrawn,  and  the  copper  determined,  as  in  No.  I.  Part.  II. 
p.  72,  by  precipitation  with  soda.  In  a  second  portion  of  the 
solution  determine  the  amount  of  metal  by  precipitation  with 
metallic  zinc.  Transfer  800  c.c.  to  a  porcelain  basin,  add 
an  excess  of  pure  sulphuric  acid,  and  evaporate  to  dryness  to 
expel  nitric  acid.  Dissolve  the  copper  sulphate  in  a  small 
quantity  of  water,  decant  the  solution  into  a  weighed  platinum 
dish,  and  place  in  it  a  piece  of  pure  zinc  (about  i  or  2  grams 
will  be  sufficient) ;  add  a  few  drops  of  hydrochloric  acid,  and 
cover  the  dish  with  a  watch-glass.  In  about  an  hour  the 
whole  of  the  copper  will  be  precipitated,  partly  as  a  coherent 
film  on  the  dish,  and  partly  in  red,  spongy  masses.  A  drop 
or  two  of  the  supernatant  fluid  should  be  tested  with  sulphu- 
retted hydrogen  water ;  it  should,  of  course,  remain  colourless. 
Assure  yourself  that  the  whole  of  the  zinc  is  dissolved,  press 
the  spongy  masses  of  copper  together,  decant  the  colourless 
liquid,  and  repeatedly  wash  the  metal  with  boiling  water 
until  the  washings  give  no  opalescence  when  tested  with 
silver  nitrate  or  barium  chloride.  Allow  the  water  to  drain 
away,  and  cover  the  copper  with  a  small  quantity  of  strong 
alcohol.  Pour  this  away,  and  dry  the  copper  in  the  steam- 
bath.  The  precipitation  of  the  copper  may  also  be  effected 
in  a  porcelain  or  glass  dish;  in  this  case  the  process 
requires  longer  time,  owing  to  the  absence  of  the  galvanic 
action  between  the  platinum  and  zinc.  A  weighed  piece  of 
platinum-foil  placed  in  the  dish,  and  of  course  weighed  with 
it  at  the  termination  of  the  experiment,  accelerates  the 
operation. 

The  filtrates  from  the  sulphides  of  lead  and  copper  con- 
tain the  iron  and  zinc ;  they  are  concentrated  to  a  small 


io6  Quantitative  Chemical  Analysis. 

bulk  (about  70  cubic  centimetres),  filtered  into  a  small  flask, 
with  a  drop  or  two  of  nitric  acid  to  oxidise  the  iron,  heated 
for  a  few  minutes,  allowed  to  cool,  and  mixed  with  a  small 
quantity  of  freshly  precipitated  barium  carbonate  suspended 
in  water.  The  liquid  should  not  contain  too  much  free  acid ; 
if  a  large  excess  is  present,  it  must  be  removed  by  adding 
sodium  carbonate  before  mixing  with  the  barium  carbonate. 
The  flask  is  closed  and  occasionally  shaken.  After  standing 
a  few  hours  the  iron  is  all  precipitated.  The  liquid  is 
filtered,  ammonium  chloride  added,  and  the  zinc  precipitated 
by  sulphuretted  hydrogen.  The  zinc  sulphide  is  filtered  off, 
washed,  re-dissolved  in  nitric  acid,  the  solution  boiled,  and 
the  zinc  re-precipitated  as  carbonate  by  the  addition  of 
sodium  carbonate.  The  zinc  carbonate  is  filtered,  washed, 
dried,  and  ignited  in  a  porcelain  crucible,  and  weighed  as 
oxide. 

The  barium  carbonate  precipitate  mixed  with  the  iron  is 
dissolved  in  hydrochloric  acid,  ammonium  chloride  is  added, 
together  with  a  slight  excess  of  ammonia,  and  the  liquid 
heated  and  filtered.  The  washed  precipitate  is  re-dissolved 
in  a  few  drops  of  hydrochloric  acid,  and  the  iron  again  pre- 
cipitated by  ammonia  free  from  carbonate,  washed,  dried, 
ignited,  and  weighed  as  ferric  oxide. 


XIV.    GERMAN  SILVER. 
(Separation  of  Copper •,  Zinc,  and  Nickel. ) 

Weigh  out  about  i  "5  gram  of  the  finely-powdered  alloy, 
and  dissolve  in  nitric  acid  at  a  gentle  heat,  with  the  precau- 
tions mentioned  in  No.  XIII.  Evaporate  the  excess  of  acid, 
and  separate  any  oxide  of  tin  which  may  be '  formed.  Pre- 
cipitate the  copper  by  sulphuretted  hydrogen  in  hot  solution : 
re-dissolve  the  copper  sulphide,  and  again  precipitate  with 
sulpuretted  hydrogen  to  separate  the  small  quantity  of 


German  Silver.  107 

co-precipitated  zinc.     The  sulphide  is  then  treated  as  in 
No.  XIII. ,  and  the  copper  weighed  as  oxide. 

To  the  filtrate  containing  the  zinc  and  nickel  is  added  a 
solution  of  sodium  carbonate  until  a  slight  permanent  pre- 
cipitate is  formed,  which  is  then  re-dissolved  by  the  cautious 
addition  of  a  few  drops  of  hydrochloric  acid.  Pass  sul- 
phuretted hydrogen  through  the  liquid,  and  to  ensure  the 
complete  precipitation  of  the  zinc  add  a  few  drops  of  a  very 
dilute  solution  of  sodium  acetate,  and  again  treat  with  sul- 
phuretted hydrogen.  Allow  to  stand  for  twelve  hours,  and 
filter  off  the  zinc  sulphide ;  wash  it  with  sulphuretted  hy- 
drogen water,  dissolve  in  hydrochloric  acid,  and  precipitate 
with  sodium  carbonate,  filter,  wash,  dry,  and  ignite  and  weigh 
as  zinc  oxide.  Boil  the  filtrate  containing  the  nickel  after 
the  addition  of  a  few  drops  of  hydrochloric  acid,  and  pre- 
cipitate with  caustic  soda  (best  in  a  porcelain  basin),  filter 
off  the  nickel  hydrate,  wash,  dry,  and  ignite  and  weigh  as 
nickel  oxide. 

XV.  BRITANNIA  METAL. 
(Separation  of  Tin  and  Antimony?) 

About  i  *5  gram  of  the  alloy,  as  finely  divided  as  possible, 
is  oxidised  in  a  porcelain  basin  with  strong  pure  nitric  acid, 
and  evaporated  to  perfect  dryness.  The  dried  mass  is  then 
washed  into  a  silver  basin,  again  evaporated  to  dryness,  and 
fused  with  an  excess  of  sodium  hydrate  (about  eight  times 
the  bulk).  It  is  treated  with  a  small  quantity  of  water,  and 
the  liquid  mixed  with  about  one-third  of  its  volume  of  strong 
alcohol.  The  stannate  of  soda,  together  with  the  excess 
of  hydrate,  is  thus  separated  from  the  sodium  antimoniate. 
The  liquid  is  allowed  to  stand  for  six  hours,  filtered,  and  the 
precipitate  washed  first  with  weak  spirit,  and  afterwards  with 
strong  alcohol.  The  antimoniate  is  dried,  transferred  to  a 
porcelain  crucible,  and  fused  with  potassium  cyanide. 
Metallic  antimony  is  thus  obtained,  which  can  be  washed 


IO8  Quantitative  Chemical  Analysis. 

from  adhering  salts,  dried,  and  weighed.  The  nitrate  con- 
taining the  tin  is  boiled  to  expel  the  alcohol,  diluted  if 
necessary,  acidulated  with  dilute  sulphuric  acid,  and  precipi- 
tated by  sulphuretted  hydrogen.  The  tin  sulphide  is 
filtered  off,  washed,  dried,  and  transferred  to  a  weighed 
porcelain  crucible,  and  cautiously  roasted  to  oxide,  and 
weighed. 

XVI.     TYPE-METAL. 
(Separation  of  Lead,  Antimony,  and  Tin.) 

The  alloy  in  fine  powder  is  treated  with  nitric  acid,  and 
tartaric  acid  is  added  to  the  solution.  The  lead  dissolves 
completely,  together  with  the  greater  part  of  the  antimony. 
The  precipitated  oxides  are  filtered  off,  and  separated,  as 
in  No.  XV.  The  solution  containing  the  lead  is  evaporated 
to  dryness  with  dilute  sulphuric  acid,  and  the  lead  sulphate 
separated  as  in  No.  XIII.  The  antimony  and  traces  of  lead 
in  the  filtrate  are  precipitated  by  sulphuretted  hydrogen, 
the  sulphide  filtered  off,  washed  into  a  flask,  and  mixed  with 
an  excess  of  yellow  sodium  or  potassium  sulphide.  The  flask 
should  be  closed  with  a  good  cork,  and  the  solution  kept 
at  a  gentle  heat.  Pour  the  clear  liquid  through  a  filter,  and 
repeat  the  digestion  with  the  alkaline  sulphide  twice.  The 
residue  consists  of  lead  sulphide,  which  may  also  contain 
copper  sulphide  :  these  are  separated  as  in  No.  XIII.  Add 
hydrochloric  acid  to  the  alkaline  filtrate,  until  the  solution 
is  distinctly  acid ;  allow  the  liquid  to  stand,  and  filter  off  the 
re-precipitated  antimony  sulphide.  This  is  dried,  transferred 
to  a  weighed  porcelain  crucible,  moistened  with  strong  nitric 
acid,  and  treated  with  ten  times  its  weight  of  fuming  nitric 
acid.  The  acid  boiling  at  86°  must  be  employed  for  this 
purpose  :  nitric  acid  of  sp.  gr.  i  -42  is  not  able  to  effect  the 
complete  oxidation  of  the  sulphur,  as  its  boiling  point  is 
about  1 6°  higher  than  the  fusing  point  of  sulphur ;  by  heating 
with  this  acid  the  separated  sulphur  fuses,  and  forms  little 


Fusible  Metal.  109 

globules  which  resist  oxidation.  The  white  mass  in  the 
crucible  consists  of  antimonic  acid  and  sulphuric  acid  :  by 
ignition  it  is  converted  into  antimony  tetroxide  Sb2  O4.  If 
the  amount  of  sulphur  mixed  with  the  precipitated  antimony 
sulphide  is  considerable,  it  is  advisable,  before  proceeding 
to  oxidise  with  nitric  acid,  to  remove  the  greater  portion  of 
it  by  treatment  with  carbon  bisulphide. 


XVII.     FUSIBLE  METAL. 

(Separation  of  Bismuth,  Lead,  and  Tin,  with  traces  of  Copper ; 
Iron,  and  Zinc.) 

The  alloy  in  the  state  of  powder  is  oxidised  with  nitric 
acid,  and  the  mass  repeatedly  digested  (three  or  four  times) 
with  an  excess  of  ammonia  and  yellow  ammonium  sulphide. 

The  mixture  should  be  kept  in  a  closed  flask,  and  the 
solution  gently  warmed.  In  presence  of  copper,  potassium 
sulphide  must  be  used,  as  the  sulphide  of  that  metal  is 
slightly  soluble  in  ammonium  sulphide.  The  tin  is  dissolved ; 
the  bismuth  and  lead,  together  with  the  traces  of  copper, 
iron,  and  zinc,  remain  undissolved.  The  liquid  is  filtered, 
and  the  tin  precipitated  as  sulphide  by  hydrochloric  acid  : 
it  is  filtered  off,  washed,  dried,  and  roasted  in  a  weighed 
porcelain  crucible  to  the  state  of  oxide.  The  sulphides  of 
bismuth  and  lead,  together  with  the  small  quantities  of 
copper,  iron,  and  zinc,  are  dissolved  in  nitric  acid,  evapo- 
rated nearly  to  dryness,  water  added,  and  the  solution,  with- 
out filtering,  again  treated  with  sulphuretted  hydrogen.  The 
lead  and  bismuth  and  traces  of  copper  are  thus  once  more 
precipitated  as  sulphides ;  the  zinc  and  iron  remain  in  solu- 
tion. The  sulphides  are  next  dissolved  in  nitric  acid, 
sulphuric  acid  is  added,  the  solution  is  evaporated  to  dryness, 
and  the  lead  separated  as  sulphate.  Nearly  neutralise  the 
solution  by  the  cautious  addition  of  ammonia,  add  a  clear 
solution  of  common  salt,  and  a  large  quantity  of  water. 


IIO  Quantitative  Chemical  Analysis. 

Allow  it  to  stand  twenty-four  hours,  and  test  the  supernatant 
liquid  by  adding  a  few  drops  of  water  :  it  ought  to  remain 
perfectly  clear.  The  bismuth  is  thus  completely  precipitated 
as  basic  chloride  (BiCIO,  or  BiCl3.  Bi2O3).  It  is  filtered 
off,  washed  with  cold  water,  dried,  and  fused  in  a  capacious 
porcelain  crucible  with  five  times  its  weight  of  potassium 
cyanide.  The  fused  mass  is  treated  with  water,  when  the 
metallic  bismuth  is  left  behind.  The  grains  of  the  reduced 
metal  are  washed  with  water,  and  afterwards  with  spirits  of 
wine,  dried,  and  weighed.  Separate  the  copper  in  the 
filtrate  from  the  precipitated  basic  chloride  of  bismuth  by 
sulphuretted  hydrogen  ;  redissolve  the  copper  sulphide,  and 
precipitate  as  oxide  by  sodium  hydrate.  The  iron  and  zinc 
are  separated  as  in  No.  XIII. 


PART    III. 

SIMPLE     VOLUMETRIC    ANALYSIS     OF    SOLIDS 
AND   LIQUIDS. 

WE  have  already  indicated  the  principle  of  this  mode  of 
analysis,  in  showing  how  it  is  possible  to  determine  the 
amount  of  silver  in  a  liquid  by  the  aid  of  a  solution  of 
hydrochloric  acid,  or  of  sodium  chloride,  of  known  strength ; 
and  we  have  also  shown  how  we  can  ascertain  the  amount 
of  alkali  in  a  solution  of  sodium  or  potassium  hydrate  by 
the  use  of  litmus  tincture,  and  of  an  acid  solution  of  known 
chemical  power.  The  following  examples  will  serve  to 
render  the  principles  of  volumetric  analysis  still  clearer. 

If  we  dissolve  a  small  piece  of  iron-wire  in  dilute  sul- 
phuric acid,  we  obtain  a  solution  of  ferrous  sulphate  which 
is  almost  colourless,  or  which  at  most  possesses  a  faint 
green  tinge.  If  we  add  to  the  solution  some  substance 


Volumetric  A  nalysis.  Ill 

which  readily  parts  with  its  oxygen,  the  colour  will  change, 
the  greenish  tinge  will  give  place  to  yellow,  the  ferrous 
salt  becoming  oxidised  to  the  state  of  ferric  oxide. 

2FeO   +   O     =     Fe2O3. 

A  few  grams .  of  potassium  permanganate  (KMnO4)  dis- 
solved in  water  give  a  deeply-coloured  purple  solution. 
Potassium  permanganate,  when  in  solution,  very  readily 
parts  with  its  oxygen  ;  if  we  add  a  few  drops  of  the 
liquid  to  the  solution  of  ferrous  sulphate,  containing  free 
sulphuric  acid,  we  notice  that  the  colour  of  the  permanga- 
nate is  instantly  discharged.  If,  however,  we  continue  to 
add  successive  quantities  of  the  permanganate  we  arrive  at 
a  point  when  its  colour  is  persistent.  Let  us  consider  what 
is  the  nature  of  this  reaction.  The  potassium  permanganate, 
in  presence  of  free  sulphuric  acid,  is  decomposed  j  perman- 
ganic acid  is  liberated,  and  potassium  sulphate  is  formed. 
The  permanganic  acid,  however,  in  presence  of  the  ferrous 
sulphate  and  free  sulphuric  acid,  readily  parts  with  its 
oxygen,  converting  the  ferrous  salt  into  the  state  of  ferric 
sulphate,  and  is  itself  reduced  to  the  state  of  manganese 
sulphate,  which  in  solution  is  colourless.  So  long  as  any 
ferrous  sulphate  remains  in  solution,  this  decolourising  action 
will  continue  ;  immediately,  however,  that  the  whole  is  con- 
verted into  ferric  sulphate,  the  red  colour  of  the  perman- 
ganic acid  will  remain  unchanged. 

This  reaction  may  be  represented  by  the  equation  : — 

ioFeSO4  +   8SO4H2  +   2KMnO4     =     5Fe2(SO4)3 
+   K2SO4  +   2MnSO4  +   8H2O. 

If  we  know  the  strength  of  the  solution  of  permanganate — 
that  is  to  say,  if  we  determine  the  number  of  cubic  centi- 
metres we  require  to  add  to  a  solution  containing  a  known 
weight  of  iron  as  ferrous  sulphate,  before  the  solution  is  per- 
manently coloured — we  can  employ  this  solution  of  perman- 
ganate to  determine  the  amount  of  iron  in  any  given  solu- 


112  Quantitative  Chemical  A  nalysis, 

tion.  Let  us  suppose  that  we  required  to  add  50  cubic 
centimetres  of  permanganate  solution  to  0-5  gram  of  iron, 
dissolved  in  dilute  sulphuric  acid,  before  the  colour  was  per- 
sistent; then  each  cubic  centimetre  of  the  permanganate 
would  be  equivalent  to  o'oi  gram  of  iron.  If  now  we 
added  the  permanganate  to  a  solution  containing  iron,  say 
from  an  iron- ore,  and  found  that  we  needed  25  cubic  centi- 
metres before  the  colour  was  permanent,  we  should  say 
that  the  amount  of  iron  in  solution  was  0-25  gram. 

Instead  of  potassium  permanganate,  we  may  employ 
potassium  bichromate  as  an  oxidising  agent.  The  reaction 
which  occurs  with  this  reagent  may  be  thus  represented  : — 

6FeSO4  +   K2Cr207  +   7SO4H2     =     3Fe2(SO4)3 
+   Cr2(S04)3  +   K2S04  +   7H20. 

This  equation  tells  us  that  294*4  parts  of  potassium 
bichromate  will  convert  6  eq.  or  336  parts  of  iron  from  the 
state  of  ferrous  to  that  of  ferric  oxide.  Potassium  bichro- 
mate possesses  a  bright  orange-red  colour  in  solution,  and 
if  the  products  of  its  reaction  on  ferrous  sulphate  were 
colourless,  we  might  continue  to  add  the  solution  of  bichro- 
mate until  its  colour  was  permanent,  when  we  should  know 
that,  as  the  chromic  acid  was  no  longer  decomposed,  the 
whole  of  the  ferrous  salt  was  changed  to  the  state  of 
ferric  salt.  Unfortunately,  however,  the  chromic  sulphate 
Cr2(SO4)3  which  is  produced  has  a  deep  green  colour  in 
solution  which  entirely  masks  the  tint  of  the  bichromate. 
We  are  accordingly  obliged  to  have  recourse  to  some  other 
method  than  the  persistency  of  the  orange  colour,  to  enable 
us  to  know  when  the  whole  of  the  ferrous  oxide  is  converted 
into  ferric  oxide.  Ferrous  salts  give  a  deep  blue  precipitate 
or  colouration  with  a  solution  of  ferricyanide  of  potassium ; 
ferric  salts  produce  no  such  colouration.  If  then  we  sprinkle 
a  few  drops  of  the  ferricyanide  solution  on  a  white  surface, 
and  from  time  to  time  take  out  a  drop  of  the  solution  of 
iron  which  is  undergoing  oxidation,  the  gradual  diminution 


Volumetric  Analysis.  113 

in  the  intensity  of  the  blue  colour  will  inform  us  of  the 
progress  of  the  reaction,  and  its  cessation  will  tell  us  when 
the  oxidation  is  complete. 

A  consideration  of  these  cases  will  enable  us  to  lay  down 
the  conditions  required  in  a  volumetric  process.  In  the  first 
place,  the  reaction  which  constitutes  the  basis  of  the  method 
must  be  constant,  even  under  a  diversity  of  circumstances. 
If,  for  example,  it  is  modified  by  the  concentration  of  the 
fluids,  or  the  amount  of  free  acid  present,  or  if  precipitates 
are  formed  during  the  reaction  of  variable  composition,  or 
if  the  presence  of  the  air  seriously  affects  the  process,  the 
reaction  cannot,  except  in  very  special  cases,  afford  the 
basis  of  a  trustworthy  method.  A  volumetric  process 
further  necessitates  that  we  possess  accurate  means  of  deter- 
mining the  completion  of  the  reaction.  Thus  the  cessation 
of  a  precipitate  in  the  case  of  the  silver-salt,  and  standard 
solution  of  sodium  chloride,  denotes  that  the  whole  of  the 
silver  is  precipitated.  The  change  of  the  litmus  tincture 
from  blue  to  red  indicates  that  the  alkali  is  neutralised. 
The  persistency  in  the  colour  of  the  permanganate  solution 
tells  us  that  the  whole  of  the  iron  is  in  the  state  of  ferric 
salt.  In  the  case  of  the  bichromate,  we  learn  the  same  fact, 
from  the  non-formation  of  a  blue  colour  with  potassium 
ferricyanide.  A  final  reaction  must  be  sensitive,  rapid,  and 
decisive  in  its  changes ;  if  it  requires  considerable  time,  or  a 
large  expenditure  of  the  testing  fluid,  or  if  it  involves  the 
passage  through  a  series  of  closely-related  tints  or  changes 
of  colour,  it  cannot  well  serve  to  indicate  the  termination  of 
the  intended  decomposition. 

In  order  to  carry  out  a  volumetric  process,  we  require  : — 

1.  A  solution  of  the  reagent  of  known  chemical  strength  : 
this  we  call  a  standard  solution. 

2.  The  means  of  accurately  determining  the  completion  of 
the  reaction. 

3.  Accurate  measuring  vessels  (pipettes,  litre-flasks,  &c.), 
and  a  graduated  instrument  termed  a  burette,   for  pouring 

i 


114 


Quantitative  CJiemical  A  nalysis. 


determinate  quantities  of  the  standard    solution   into  the 
liquid  on  which  it  is  to  act. 

The  amount  of  apparatus  specially  required  for  volumetric 
analysis  is  not  very  extensive.  In  addition  to  a  few  beakers, 
flasks,  porcelain  basins,  glass  stirrers,  &c.,  the  student  must 


FIG.  36. 


provide  himself  with  a  set  of  measuring  flasks,  pipettes, 
and  burettes. 

The  most  convenient  series  of  measuring  flasks  is  the 
following : — 

(i)  1,000  CD.C.J  (2)  500 c.c.  ;  (3)  300C.C.;  (4)  2500.0.;  and 


Graduation  of  Vessels.  115 

(5)  looc.c.  They  should  be  fitted  with  well  ground  glass 
stoppers,  and  the  graduation  mark  of  each  should  be  near 
the  middle  of  the  neck.  The  space  between  the  mark  and 
stopper  allows  the  fluid  to  be  more  readily  mixed  by  agitation. 
The  flasks  should  be  sufficiently  thin  to  be  heated  without 
risk  of  fracture.  Fig.  36  d  represents  a  convenient  form  of 
litre-flask. 

The  following  is  the  most  convenient  series  of  pipettes  : — 
(i)  100  c.c.  ;  (2)  50  c.c. ;  (3)  25  c.c.  ;  (4)  10  c.c. ;  and  (5) 
5  c.c.  Several  i  c.c.  pipettes  will  be  also  needed ;  these 
are  readily  made  from  glass  tubing.  The  pipettes  have  the 
form  seen  in  fig.  36  bb. 

The  measuring  flasks  and  pipettes  are  generally  sold  with 
the  graduating  marks,  their  denomination,  and  the  temperature 
at  which  the  graduation  is  effected,  etched  upon  them.  But 
before  employing  them  the  operator  must  never  neglect  to 
verify  their  capacities.  It  must  be  borne  in  mind  that 
pipettes  are  to  be  graduated  to  deliver  their  contents ; 
measuring  flasks,  however,  should  be  graduated  both  to 
contain  and  to  deliver.  A  50  c.c.  pipette,  accordingly,  needs 
to  hold  more  than  50  c.c.  of  liquid  ;  it  must  hold  this 
quantity //w  that  amount  which  adheres  to  the  glass  when 
the  liquid  is  allowed  to  flow  out.  We  frequently  use  the 
measuring  flasks  to  dilute  liquids  to  determinate  volumes, 
from  which  we  afterwards  withdraw  aliquot  portions  by 
means  of  the  pipettes  ;  occasionally,  however,  it  is  necessary 
to  transfer  a  determinate  volume  of  fluid  from  the  flask ;  it 
is  desirable,  therefore,  that  the  same  flask  should  have  a 
double  graduation — one  to  contain,  the  other  to  deliver. 

1,000  c.c.  of  distilled  water  at  4°  C.  weigh  1,000  grams. 
If,  therefore,  we  place  the  litre-flask,  perfectly  clean  and 
dry,  on  one  pan  of  a  balance  capable  of  turning  with  0-05 
gram  when  carrying  2  kilos,  and  tare  it,  placing  1,000 
grams  on  the  weight-pan,  and  pouring  in  water  of  4°  C.  until 
the  equilibrium  is  established;  the  level  of  the  water 
will  indicate  to  us  the  proper  position  of  the  graduating 

I  2 


Quantitative  Chemical  A  nalysis. 


line.  The  flask  contains  1,000  c.c.  of  liquid  when  filled  up  to 
that  line.  If  we  now  pour  out  the  water,  allow  the  flask  to 
drain  for  a  few  seconds,  remove  the  1,000  grams  from  the 
weight-pan,  and  re-adjust  the  tare  of  the  flask,  replace  the 
1,000  grams,  and  again  fill  up  the  flask  with  water  at  4°, 
until  the  equilibrium  is  again  established,  the  level  of  the 
water  will  now  indicate  to  us  the  position  of  what  we  may 
call  the  delivery-mark.  The  flask  filled  up  to  this  mark  and 
emptied,  delivers  1,000  c.c.  But  a  very  superficial  observance 
of  the  surface  of  the  liquid  in  the  neck  of  the  flask  shows  us 
that  it  is  not  perfectly  horizontal.  Unless,  therefore,  we  in- 
variably make  some  determinate  point  of  the  curve  to  coin- 
cide with  the  graduating  line,  our  measurements  will  not  be 
uniform.  It  will  be  found  most  convenient  to  take  the 
lowest  point  of  the  curve  or  meniscus  as  the  fixed  point.  In 
verifying  or  correcting  the  graduation  of  the  flask,  the  true 
mark  is  scratched  with  a  diamond  so  as  to  coincide  with  the 
lowest  point  of  the  curve  of  water  in  the  neck  ;  and  when  it 
is  desired  that  the  flask  shall  be  filled  with  1,000  c.c.,  the 
liquid  is  to  be  poured  in  until  the  lowest  portion  of  its  sur- 
face exactly  reaches  this  position. 

The  distilled  water  in  a  laboratory  has  very  seldom  a 
temperature  of  4°,  but  as  we  know  from  experiment  the  rate 
at  which  the  liquid  expands,  it  is  easy  to  calculate  what  would 
be  the  weight  of  1,000  c.c.  at  any  given  temperature.  This 
weight  may  be  obtained  from  the  following  table. 

The  weight  of  1,000  c.c.  of  water  of  t°  C.,  when  determined 
by  means  of  brass  weights  in  air  of  o°  C.,  and  of  a  tension 
07 6m.,  is  equal  to  1,000—3:  grams.* 


t° 

o 

I 

2 

3 

4 

5 

6 

7 

8 

9 

X 

1-25 

1-20 

I'lS 

1-13 

I'I2 

I'I2 

1-14 

1-16 

I  '21 

1-27 

*  Watts's  'Dictionary  of  Chemistry, '  vol.  i.  p.  256. 


Graduation  of  Pipettes. 


117 


t° 

IO 

II 

12 

13 

1.4 

15 

16 

17 

18 

19 

2'55 

X 

1-34 

i'43 

I  '52 

I-63 

I-76 

I-89 

2-04 

2-20 

2-37 

t° 

20 

21 

22 

23 

24 

25 

26 

27 

28 

29 

X 

274 

2'95 

3-17 

3'39 

3^3 

3-88 

4'i3 

4'39 

4-67 

4  '94 

The  student  is  now  in  possession  of  all  the  data  required 
to  graduate  his  measuring  vessels.  He  should  fill  a  large 
beaker  with  distilled  water,  place  it  in  the  balance-room,  and 
ascertain  its  temperature.  Let  us  suppose  that  it  is  15°,  and 
that  he  requires  to  graduate  his  litre-flask  to  contain  1,000 
c.c.  On  reference  to  the  table,  we  see  that  the  value  of  x 
corresponding  to  15°  is  1*89;  accordingly,  the  weight  of 
water  necessary  to  be  poured  into  the  flask  is  1,000— 1*89= 
998-11  grams.  In  graduating  the  250  c.c.  flask,  he  would 
of  course  take  one-fourth  of  this  amount,  viz.,  249-53  grams. 
He  places  998-1,  or  249-5  grams,  as  the  case  may  be,  on  the 
weight-pan,  in  addition  to  the  tare  of  the  flask,  and  fills  up 
the  flask  with  water  until  it  is  exactly  equipoised.  He  then 
marks  with  a  diamond  on  the  neck  of  the  flask  the  position 
of  the  lowest  point  of  the  meniscus.  He  now  repeats  the 
observation  in  the  manner  already  described  in  order  to  obtain 
the  graduation  for  delivery. 

He  next  proceeds  to  re-graduate  his  pipettes.  The  light 
frame  A  B  (fig.  37),  made  of  stout  brass  wire,  carries  two  clips 
of  thin  sheet  brass  closed  by  sliding  collars  ;  through  the 
lower  clip  is  inserted  the  upper  end  of  the  pipette  to  be 
graduated :  this  is  connected  by  caoutchouc  tubing  with  the 
glass  stopcock  c,  to  which  a  short  length  of  thermometer 
tube  can  be  attached,  as  shown  in  the  figure.  To  begin  with, 
the  thermometer  tube  is  removed,  and  a  piece  of  wider  glass 
tubing  placed  in  the  caoutchouc  tubing,  and,  the  stopcock 
being  opened,  the  pipette  is  filled  by  suction  with  distilled 


118 


Quantitative  Chemical  Analysis. 


FIG.  37. 


water  a  centimetre  or  so  above  the  mark.  The  object  of 
the  glass  tube  is  to  prevent  the  caoutchouc  being  moistened 
by  the  lips.  The  end  of  the  pipette  which  has  been  dipped 
beneath  the  surface  of  the  water  is  dried  by  a  cloth,  the  stop- 
cock is  reopened,  and  the  water  is  allowed  to  flow  out  again 
by  its  own  weight  into  the  beaker. 
As  soon  as  the  flow  of  water  has 
ceased,  the  pipette  is  held  vertically 
for  three  or  four  seconds  to  allow 
the  liquid  adhering  to  the  glass  to 
flow  down  into  the  stem;  the  end 
is  then  caused  to  touch  the  surface 
of  the  water.  Of  the  various  me- 
thods of  delivering  pipettes,  this  is 
most  accurate.  The  glass  tube  is 
withdrawn  from  the  caoutchouc,  and 
the  short  length  of  thermometer 
tubing,  the  end  of  which  is  drawn 
out  before  the  lamp  so  as  to  make 
the  bore  of  very  small  diameter,  is 
placed  in  its  stead.  The  whole  is 
then  suspended  from  the  arm  of  a 
balance  turning  with  0*05  gram,  in 
the  manner  represented  in  fig.  37, 
and  accurately  counterpoised.  The 
pipette  is  removed  from  the  balance, 
the  thermometer-tube  is  withdrawn, 
and  the  wide  glass  tube  reinserted ; 
the  cock  is  opened,  and  the  water 
is  again  drawn  into  the  pipette 
one  or  two  centimetres  above  the 

mark  already  etched  upon  the  stem.  The  end  of  the  pipette 
is  again  wiped  with  a  dry  cloth,  the  glass  tube  is  replaced  by 
the  thermometer  tubing,  and  the  pipette  is  again  suspended 
from  the  balance  arm.  The  temperature  of  the  water  is  ob- 
served j  suppose  it  to  be  15°,  and  that  the  pipette  is  to  deliver 
50  c.c.,  we  find  from  the  table  that  the  weight  of  water  possess- 


Graduation  of  Pipettes.  1 19 

ing  this  volume  is    ?  — ^-  =  49  -90  grams.    This  weight 

20 

is  accordingly  placed  on  the  weight-pan,  in  addition  to  the 
tare,  and  the  balance  is  caused  to  oscillate.  In  all  proba- 
bility the  pipette  and  its  contents  will  be  too  heavy  ;  the 
cock  is  now  opened,  and  one  or  two  drops  of  water  are 
allowed  to  flow  out  into  a  beaker  placed  below.  On 
account  of  the  slowness  with  which  the  air  finds  it  way 
through  the  narrow  bore  of  the  thermometer-tube,  the 
number  of  drops  may  be  regulated  with  great  nicety. 
Successive  drops  are  thus  allowed  to  flow  out  until  the 
balance  is  in  equilibrium.  The  lowest  part  of  the  meniscus 
is  then  marked  on  the  stem.  The  pipette  will  now  deliver 
50  c.c.,  if  emptied  in  the  manner  described.  The  determi- 
nation should  be  repeated  ;  if  made  with  proper  care,  the 
level  obtained  in  the  second  experiment  will  be  identical 
with  that  found  in  the  first.  The  capacities  of  the  remain- 
ing pipettes  are  verified  in  the  same  manner. 

The  Burette. — This  instrument  serves  to  deliver  definite 
volumes  of  the  standard  solutions.  Various  forms  of  the 
burette  have  been  devised,  but  the  most  convenient  modifi- 
cations are  those  of  Gay-Lussac  and  Mohr.  Gay-Lussac's 
burette  is  seen  in  fig.  36  c.  It  consists  of  a  tube  about 
30  centimetres  long,  and  1-4  to  1-8  centimetres  wide,  sealed 
at  one  end,  and  furnished  with  a  narrow  side-tube,  starting 
near  the  bottom,  and  running  close  to  the  side,  to  within 
about  2  centimetres  from  the  open  end,  where  it  is  bent 
slightly  in  the  manner  seen  in  the  figure.  These  burettes 
are  usually  made  in  two  sizes — one  to  hold  25  c.c.,  and 
graduated  in  ^  c.c.  ;  the  other  to  hold  50  c.c.,  and  gradu- 
ated in  i  c.c.  They  are  graduated  'for  delivery.'  The 
correctness  of  the  graduation  should  be  tested  previous  to 
use,  by  filling  the  burette  with  distilled  water,  and  emptying 
it  through  the  side-tube,  until  the  bottom  of  the  meniscus 
is  coincident  with  the  lowest  division.  The  temperature  of 
the  water  is  then  ascertained,  the  burette  is  tared,  and  the 


120  Quantitative  Chemical  Analysis. 

weight  of  the  water  supposed  to  be  required  to  fill  it  is 
placed  on  the  weight-pan,  and  the  instrument  is  filled  up 

FIG.  39. 


FIG.  38. 


with  the  distilled  water 
until  the  balance  is  in  equi- 
librium ;  if  the  meniscus  is 
now  coincident  with  the 
zero  point,  the  instrument 
is  correctly  graduated.  Dif- 
ferences of  less  than  0-05 
c.c.  may  generally  be  neg- 
lected. So  long  as  its  bore 
is  uniform,  and  the  divisions 
are  of  an  invariable  width, 
the  instrument  need  not  be 

discarded,  even  if  the  lowest  point  of  the  meniscus  is  not 
coincident  with  the  zero.  Let  us  suppose  that  on  a  50  c.c. 
burette,  on  which  250  divisions=5o  c.c.,  the  lowest  point  of 


Graduation  of  Burettes. 


121 


FIG.  40. 


the  meniscus  was  coincident  with  the  division  correspond- 
ing to  0-8  c.c.,  then  obviously  250—4,  or  246  divisions,  are 
equivalent  to  50  c.c.,  and  i  di vision =2%  c.c.,  or  0-203  c-c- 
Accordingly,  the  indications  of  the  burette  must  be  multi- 
plied by  i  'oi  6,  to  give  the  correct  number  of  cubic  centimetres 
delivered.  Thus,  if  in  an  analysis  we  had  delivered  ap- 
parently 25  c.c.  from  such  a  burette,  we  should  in  reality 
have  delivered  25  x  roi6  =  25-4  c.c.  of  liquid. 

In  using  the  burette,  the  edge  of  the  side-tube  should 
be  greased  slightly  ;  this  prevents  the  possibility  of  liquid 
adhering  to  the  outside  of  the  tube  when  the  burette 
is  replaced  vertically  in  its  support.  With  a  little  practice  it 
is  easy  to  deliver  the  liquid  in  a  stream  or  in  drops  ;  when 
the  burette  is  brought  to  the  vertical,  to  be  read  off,  it  is 
necessary  to  wait  for  a 
few  seconds  before 
making  the  observa- 
tion, in  order  that  the 
liquid  may  attain  a  con- 
stant level. 

Mohr's  burette  is 
seen  in  fig.  38.  It  is 
simply  a  divided  tube, 
contracted  at  its  lower 
end,  and  fitted  with  a 
short  length  of  caout- 
chouc tubing  into  which 
is  inserted  a  glass  jet. 
The  sides  of  the  caout- 
chouc tube  can  be 
pressed  together  by 
means  of  the  spring 
clamp.  This  form  of 
burette  is  not  so  gene- 
rally applicable  as  that 
of  Gay-Lussac,  since  the  caoutchouc  is  acted  upon  by 


122  Quantitative  Chemical  A  nalysis. 

several  of  the  substances  employed  in  standard  solutions.  In 
the  more  modern  form  of  the  burette,  a  glass  stop-cock  is  sub- 
stituted for  the  india-rubber  and  clamp.  This  modification 
(fig.  360)  leaves  nothing  to  be  desired.  It  is  especially  con 
venient  where  a  great  number  of  analyses  of  the  same  kind 
have  to  be  made,  as  in  metallurgical  laboratories,  chemical 
works,  &c.  In  such  cases  the  burette  may  be  conveniently 
arranged  as  shown  in  fig.  39.  The  bottle  A  contains  the 
standard  solution ;  on  opening  the  clamp  a,  the  liquid  fills 
the  burette  gradually,  and  without  the  formation  of  air- 
bubbles.  Fig.  40  shows  another  method  of  connecting  the 
burette  with  the  reservoir  of  the  standard  solution.  The 
liquid  is  driven  into  the  burette  by  simply  blowing  through 
the  caoutchouc  tube  a.  The  graduation  of  Mohr's  burette 
may  be  verified  by  filling  the  instrument  with  water,  and 
allowing  successive  quantities  of,  say,  10  c.c.,  to  flow  out 
into  a  weighed  beaker.  If  the  10  c.c.  weigh  9-98  grams,  the 
burette  is  correctly  graduated.  Of  course- due  care  must  be 
taken  to  allow  the  liquid  adhering  to  the  sides"  of  the  tube 
to  flow  down  before  the  level  is  read  off. 

The  correct  reading  off  of  the  burette  may  be  facilitated 
by  the  use  of  a  little  device  recommended  by  Mohr.  A 
broad  strip  of  black  paper  is  pasted  on  a  white  card  or 
sheet  of  white  paper,  and  this  is  held  behind  the  burette, 
so  that  the  edge  of  the  black  paper  is  about  2  mm. 
below  the  dark  zone  of  the  liquid.  The  lower  edge  of  the 
liquid  is  thus  sharply  defined,  and  may  be  read  off  with 
certainty.  A  little  caoutchouc  band,  slipped  round  the  tube, 
and  through  the  card,  renders  the  arrangement  more  con- 
venient. 

In  reading  off  the  Gay-Lussac  burette,  the  level  of  the 
liquid  should  be  brought  to  the  direct  line  of  vision.  This 
may  conveniently  be  determined  by  pasting  a  narrow  strip 
of  black  paper  upon  the  side  of  the  room,  ten  or  twelve  feet 
from  the  operator,  and  on  a  level  with  his  eye.  The  burette 
is  held  'perpendicularly  between  the  thumb  and  first  finger. 


Graduation  of  Burettes. 


123 


FIG.  41. 


in  such  position  that  the  black  strip  appears  immediately 

behind  the  level  of  the  liquid.     Greater  certainty  in  reading 

off  may  be  attained  by  the  use  of  Erdmann's  float  (fig. 

41).     It  is  simply  an  elongated  glass  bulb, 

somewhat  smaller  in  diameter  than  the  burette, 

containing  a  small  quantity  of  mercury.    The 

upper  end  is  drawn  out,  sealed,  and  bent  into 

a  little  hook,  by  which  the  bulb  can  be  lifted 

in  and  out  of  the  burette  by  the  aid  of  a  bent 

wire.     Round  the  bulb  runs  a  line  a,  etched 

by  means  of  hydrofluoric  acid,  or  scratched 

by  a  diamond.     The  coincidence  of  this  line 

with  the  division  of  the  burette  is  taken  as 

the  reading.     The  float  should  move  easily 

within  the  burette,  and  so   that  the   line  is 

always   parallel   with   the  divisions ;   by   its 

means  the  volume  of  the  liquid  delivered  may 

be  determined  to  within  0*005  c.c. 

In  certain   cases  the  quantity  of  the  liquid 
delivered   is   determined    by    weight.      The 
solution  is  contained  in   the  little   weighed 
flask  seen  in  fig.  42.     The  required  amount 
is   poured  through    the  delivery-tube, 
which  should  be  slightly  greased  at  the 
edge.  By  weighing  the  apparatus  before 
and  after  delivery,  the  amount  of  liquid 
employed  is  at  once    determined.     A 
method  of  making  a   simple  form   of 
this  apparatus  is  described  under  the 
section  '  Ash  Analysis.' 


FIG.  42. 


We  now  proceed  to  the  experimental  study  of  certain 
volumetric  processes.  We  shall  describe  here  a  few  typical 
processes  to  enable  the  student  to  familiarise  himself  with 
this  mode  of  estimation.  Other  methods  will  be  given  in 
Part  IV. 


124  Quantitative  Chemical  Analysis. 

I.  DETERMINATION  OF  CHLORINE  BY  STANDARD  SILVER- 
SOLUTIONS. 

If  we  add  silver  nitrate  to  a  solution  of  a  chloride,  say  of 
common  salt,  we  obtain  a  white  precipitate  of  silver  chloride; 
if  we  continue  to  add  the  silver  solution,  the  formation  of 
this  substance  goes  on  until  the  whole  of  the  chlorine  is 
precipitated.  If  we  add  silver  nitrate  to  a  solution  of 
potassium  chromate,  we  obtain  a  dark-red  precipitate  of 
silver  chromate.  If  now  we  mix  the  alkaline  chloride  and 
chromate  together,  and  cautiously  add,  little  by  little,  the 
silver  nitrate  solution,  we  notice  that  the  chloride  is  first  de- 
composed ;  and  white  silver  chloride  continues  to  be  formed 
so  long  as  any  chlorine  remains  in  solution.  It  is  only  after 
the  whole  of  the  chlorine  is  precipitated  that  we  observe  the 
formation  of  the  dark-red  silver  chromate.  This  principle 
constitutes  the  basis  of  an  accurate  volumetric  process.  To 
carry  it  out  we  require  a  standard  solution  of  pure  silver 
nitrate  free  from  excess  of  acid,  and  a  solution  of  potassium 
chromate. 

Preparation  of  Pure  Silver. — Chemically-pure  silver  is 
frequently  needed  in  volumetric  analysis.  We  not  only  re- 
quire it  in  the  present  process :  we  shall  have  occasion  to 
use  it  in  determining  the  strength  of  the  hydrochloric  acid 
solution  employed  in  alkalimetry.  It  is  therefore  desirable 
that  the  student  should  prepare  at  one  time  all  that  he  will 
need  in  this  and  subsequent  operations. 

About  50  grams  of  standard  silver  (composed  of  12-3  parts 
of  silver  and  i  part  of  copper)  are  dissolved  in  dilute  nitric 
acid  in  a  thin  porcelain  basin  at  a  gentle  heat  ;  the  solution 
is  evaporated  to  dryness,  and  the  residue  heated  to  fusion. 
The  cooled  mass  is  then  dissolved  in  ammoniacal  water, 
allowed  to  stand  for  a  short  time,  and  filtered  into  a  large 
flask.  The  filtrate  is  diluted  to  2\  litres.  50  c.c.  are  with- 
drawn, heated  nearly  to  boiling,  and  mixed  with  a  solution 
of  neutral  ammonium  sulphite  (prepared  by  neutralising 


Pure  Silver.  125 

ammonia  with  sulphur  dioxide  gas)  added  drop  by  drop, 
until  the  liquid  is  decolourised.  The  ammonium  sulphite  solu- 
tion is  then  mixed  in  the  proportion  demanded  by  this  trial 
with  the  2450  c.c.  of  liquid  in  the  flask,  which  is  then  closed 
air-tight.  In  about  48  hours,  nearly  a  third  part  of  the 
silver  is  deposited  as  a  crystalline  powder,  and  the  remainder 
is  thrown  down  on  heating  the  liquid  to  60°  or  70°  for  a  short 
time.  The  liquid  is  now  completely  decolourised,  unless  it 
contains  nickel  or  cobalt,  which  are  not  infrequent  impuri- 
ties in  standard  silver,  when  it  will  be  light-green  or  pink. 
The  precipitated  silver  is  washed  with  distilled  water,  and 
digested  with  strong  ammonia,  again  washed  and  dried.  It 
is  then  fused  with  about  3  grams  of  ignited  and  powdered 
borax,  previously  mixed  with  a  little  sodium  nitrate,  in  an 
unglazed  porcelain  crucible,  and  the  button  of  metal  is 
washed  with  hot  water,  and  rubbed,  if  necessary,  with  a  little 
sea-sand.  It  should  then  be  rolled  out  into  foil,  sufficiently 
thin  to  be  readily  cut  with  a  pair  of  scissors. 

Preparation  of  the  Standard  Solution  of  Silver. — 10794 
grams  of  the  foil  are  weighed  out,  placed  in  a  porcelain  basin 
provided  with  a  glass  cover,  and  dissolved  in  dilute  pure 
nitric  acid  on  the  water-bath.  When  the  whole  of  the  metal 
is  dissolved,  the  under  surface  of  the  glass  is  rinsed  into  the 
dish,  and  its  contents  are  evaporated  to  complete  dryness  on 
the  water-bath,  and  gently  heated  over  the  lamp  until  the  salt 
fuses.  The  dry  and  neutral  silver  nitrate  is  dissolved  in  pure 
water,  and  the  solution  carefully  poured  into  the  litre-flask, 
the  dish  being  repeatedly  washed  out  with  fresh  portions  of 
distilled  water.  The  flask  is  now  filled  up  to  the  containing- 
mark  with  distilled  water,  the  stopper  is  inserted,  and  the 
flask  well  agitated.  The  liquid  constitutes  a  deci-normal  solu- 
tion  of  silver  nitrate  :  i  c.c.  =  0-010794  gram  silver;  it  is 
therefore  equivalent  to  0-003546  gram  of  chlorine,  or 
•003646  gram  of  hydrochloric  acid,  or -00585  gram  of  sodium 
chloride.  The  solution  should  be  poured  into  a  perfectly 


126  Quantitative  Chemical  Analysis. 

clean  and  dry  bottle,  provided  with  a  well-fitting  stopper :  it 
should  be  labelled  '  Deci-normal  Silver  Solution.'  NOTE. — 
By  a  normal  solution  is  to  be  understood  a  solution  con- 
taining i  eq.  of  the  substance,  in  grams,  dissolved  in  1,000 
c.c.  of  liquid.  Thus  a  normal  solution  of  silver  would  con- 
tain 107 '94  grams  of  the  metal  in  i  litre  of  the  solution.  A 
normal  solution  of  hydrochloric  acid  would  contain  36-46 
grams  of  HC1  in  1,000  c.c.  A  deci-normal  solution  contains 
one-tenth  of  an  equivalent;  a  centi-normal  the  one-hundredth 
part  of  an  equivalent,  in  grams,  per  litre. 

Preparation  of  Potassium  Chr ornate  Solution. — The  com- 
FIG.  43.  mercial  salt  is  recrystallised  until  it 

is  free  from  chlorine  :  the  solution 
acidified  with  nitric  acid  should  not 
give  the  least  turbidity  on  the  addi- 
tion of  a  drop  of  silver  nitrate  solu- 
tion. Its  solution  should  be  kept 
in  a  little  bottle  A,  through  the 
cork  of  which  runs  a  narrow  tube 
with  a  mark  d  scratched  upon  it. 
This  allows  of  a  constant  quantity 
of  the  solution  to  be  withdrawn 
from  the  bottle.  (Fig.  43.) 

The  Process. — Aquantity  of  pure  sodium  chloride  (see  p.  79), 
is  powdered,  and  gently  heated,  and  whilst  warm  introduced 
into  a  small  tube,  fitted  with  a  good  cork.  About  i  gram  of 
the  chloride  is  accurately  weighed  out  into  the  \  litre  flask, 
and  dissolved  in  distilled  water ;  the  flask  is  filled  up  to  the 
containing-TC&fo,  and  the  solution  well  agitated.  The  burette 
(either  Gay-Lussac's  or  Mohr's  may  be  used)  is  rinsed  out 
with  a  little  of  the  standard  silver  solution  (which  is  thrown 
into  the  *  silver  residue '  bottle)  and  filled  up  to  the  zero  with 
the  silver  solution.  50  c.c.  of  the  solution  of  sodium  chloride 
are  withdrawn  from  the  flask,  and  run  into  a  porcelain  basin, 


Estimation  of  Chlorine.  127 

and  mixed  with  a  measure  of  the  chromate  solution ;  the 
silver  solution  is  added,  drop  by  drop,  from  the  burette, 
until  the  red  colour  of  the  silver  chromate  is  permanent 
Each  drop  of  the  silver  solution  forms  a  red  spot  in  the 
yellow  liquid,  which  quickly  disappears,  so  long  as  any  chlo- 
ride remains  in  solution  ;  immediately  all  the  chlorine 
is  precipitated,  the  red  colour  of  the  chromate  of  silver  is 
unaltered.  The  process  is  now  at  an  end.  The  volume  of 
the  silver  solution  employed  is  read  off,  corrected,  if  neces- 
sary, for  the  error  of  the  graduation  (see  p.  121),  and  o-i 
c.c.  subtracted,  this  expenditure  of  silver  being  required  to 
render  the  final  reaction  evident  The  analysis  should 
be  repeated  on  a  second  portion  of  50  c.c.  of  solution. 
An  actual  example  will  render  the  method  of  calculation 
clear.  1*0850  gram  of  pure  salt  was  dissolved  in  250  c.c. 
of  distilled  warter.  50  c.c.  of  this  solution  required  in  experi- 
ment L,  37-1  c.c.;  in  experiment  II.,  37-2  c.c.  ;  in  experiment 
III.,  37 -2  c.c.  of  silver  solution.  Mean  37-17  c.c.  Subtract  o-i 
for  final  reaction.  37*07  x  -003546  =  0-1314  gram  of 
chlorine.  50  c.c.  of  liquid  contain  0-217  gram  of  salt 

Accordingly,  the  salt  contains  —  ^=60*56  per  cent 

chlorine.     Theory  requires  60  60  per  cent. 

The  remainder  of  the  solution  of  the  sodium  chloride 
should  be  poured  into  a  clean  and  dry  stoppered  bottle,  and 
its  strength  marked  on  a  label  attached  to  the  bottle.  It 
will  be  useful  in  cases  where,  in  determining  chlorine  by 
this  method,  we  imagine  that  we  have  added  an  excess  of 
silver  solution.  We  have  only  to  add  a  definite  volume, 
say  i  c.c.  of  the  solution,  to  the  turbid  liquid,  and  after  the 
last  trace  of  silver  chromate  has  disappeared,  again  add  the 
silver  solution  until  the  final  point  is  exactly  obtained.  In 
the  case  above  cited,  50  c.c.  of  salt  solution  equal  37*07  c.c. 
of  silver  solution  ;  accordingly  i  c.c.  of  salt  =  0-7  c.c.  of 
silver.  0-7  +  0-1  (for  final  reaction)  or  0-8  c.c.  subtracted 
from  the  total  amount  of  silver  solution  employed,  gives  the 
exact  amount  used  in  the  analysis. 


128  Quantitative  Chemical  Analysis. 

II.  INDIRECT  DETERMINATION  OF  POTASSIUM  AND  SODIUM 
BY  MEANS  OF  STANDARD  SILVER  SOLUTION  AND  POTASSIUM 
CHROMATE. 

The  deci-normal  silver  solution  may  be  used  for  a  variety 
of  estimations  in  which  the  amount  of  chlorine  present  may 
be  taken  as  a  measure  of  the  other  constituents.  We  shall 
have  occasion  to  mention  several  of  the  applications  of  this 
solution  in  the  General  Part  (Part  IV.).  We  have  already 
described  the  method  of  estimating  potassium  and  sodium 
by  gravimetric  analysis :  as  an  example  of  the  above-men- 
tioned applications  of  the  solution  of  standard  silver,  we 
proceed  to  show  how  these  alkalies,  when  together,  may  be 
estimated  by  volumetric  analysis. 

From  3  to  4  grams  of  pure  Rochelle  salt  (C4H4KNaO6. 
4.H2O)  are  gently  heated  in  a  platinum  basin  nntil  the  water 
of  crystallisation  is  expelled.  The  temperature  is  then  in- 
creased until  the  mass  is  completely  carbonised  j  the  heat 
should  not  exceed  low  redness,  or  a  loss  of  alkali  will  be 
incurred.  The  alkaline  carbonates  in  the  charred  mass  are 
then  dissolved  in  a  small  quantity  of  hot  water,  filtered,  and 
the  charcoal  repeatedly  washed  with  successive  quantities  of 
water.  A  slight  excess  of  pure  hydrochloric  acid  is  added 
to  the  filtrate  contained  in  a  weighed  platinum  basin,  and 
covered  with  a  watch-glass,  and  as  soon  as  the  evolution  of 
gas  ceases,  the  under  surface  of  the  watch-glass  is  rinsed 
into  the  basin,  and  the  liquid  is  evaporated  to  complete  dry- 
ness,  and  heated  in  the  air-bath  to  180°.  The  alkaline 
chlorides  are  weighed  and  dissolved  in  a  small  quantity  of 
water,  the  solution  poured  into  a  ^-litre  flask,  and  diluted  to 
the  containing-vxsdL.  50  c.c.  are  then  withdrawn  and  titrated 
with  silver  solution  and  potassium  chromate  in  the  manner 
described.  From  the  weight  of  the  chlorides,  and  of  the 
chlorine  they  contain,  we  can  readily  calculate  the  propor- 
tion of  the  alkalies  in  the  mixture. 

Let  x  stand  for  the  potassium,  and  y  for  the  sodium,  s  for 


A  Ikalimetry.  1 29 

the  weight  of  the  mixed  chlorides,  and  A  for  that  of  the 
chlorine  found. 

[(S-A).  1-54]  --  A 

"*      ~~~ 7 

0-63 

y    -    A  -  [  (S  —  A)  0-91] 

••»- a  «»•-?  -^-a-s 

III.  ESTIMATION  OF  CHLORIC  ACID. 

Weigh  out  about  0*5  gram  of  dry  potassium  chlorate  into 
a  small  beaker  in  which  you  have  previously  placed  about 
20  grams  of  thin  sheet- zinc  covered  with  spongy  copper, 
in  the  manner  described  on  p.  96.  Add  about  25  c.c.  of 
water,  cover  the  beaker  with  a  watch-glass  and  boil  the 
liquid  gently  for  about  an  hour.  Add  water  to  the  beaker, 
filter  the  liquid*  into  a  porcelain  basin,  and  wash  the  zinc  and 
copper  in  the  beaker  repeatedly  with  hot  water.  By  the 
action  of  the  nascent  hydrogen,  the  alkaline  chlorate  is 
reduced  to  chloride.  The  nitrate  should  be  quite  neutral. 
Determine  the  amount  of  chlorine  in  the  liquid  by  standard 
silver  and  potassium  chromate  solutions,  i  c.c.  of  the  solu- 
tion is  equivalent  to  0-01226  gram  of  potassium  chlorate. 

Example. — 0-2492  gram  of  potassium  chlorate,  treated  in 
the  manner  described,  required  20*2  c.c.  of  deci-normal  silver 
solution.  20-2  x -01226=0-2477  gram  potassium  chlorate. 

IIlA.  DETERMINATION  OF  CHLORINE  IN  PRESENCE  OF 
SULPHITES. 

Add  to  the  solution  of  the  salts  a  very  slight  excess  of  a 
solution  of  potassium  permanganate  free  from  chlorine ; 
neutralise  the  liquid  with  pure  soda,  and  then  add  the 
potassium  chromate  and  standard  silver  nitrate  solution  in 
the  usual  manner.  This  preliminary  oxidation  of  the  sul- 
phurous acid  is  necessary,  otherwise  the  chromate  solution 
would  be  reduced. 


130  Quantitative  Chemical  Analysis. 

ALKALIMETRY. 

Preparation  of  Normal  Solution  of  Hydrochloric  Add. — i  c.c. 
=0-03646  gram  HC1.  Messrs.  Roscoe  and  Dittmar  have 
shown  that  if  a  solution  of  hydrochloric  acid  containing  20-2 
per  cent.  HC1  be  boiled  under  the  ordinary  pressure  of  the 
atmosphere,  the  acid  and  water  distil  over  in  the  proportion 
in  which  they  are  contained  in  the  boiling  liquid.  If  we  take 
a  solution  of  the  acid  having  approximately  this  composition 
and  boil  it  in  a  retort  until  about  half  of  it  has  distilled  over, 
we  may  be  sure  that  the  residue  contains  about  20*2  per  cent. 
of  acid.  This  principle  affords  the  basis  of  a  method  of 
preparing  a  standard  solution  of  hydrochloric  acid. 

We  commence  by  ascertaining  the  specific  gravity  of  a 
strong  solution  of  hydrochloric  acid  by  means  of  the  hydro- 
meter (see  Appendix),  and  we  then  add  water  to  it  until  its 
specific  gravity  is  reduced  to  i  *i.  A  solution  of  this  strength 
contains  about  20*2  per  cent,  of  HC1.  The  amount  of  water 
x  which  we  require  to  add  to  a  measured  quantity  of  strong 
hydrochloric  acid  A,  of  specific  gravity  a,  to  reduce  it  to  the 
specific  gravity  b  (in  this  case  i'i),  is  found  from  the  formula 

.  A  (a  -  b) 
~b-T 

Let  us  suppose  that  the  specific  gravity  of  our  acid  is  1*16: 
in  order  to  bring  its  specific  gravity  down  to  i'i  we  shall 

require  to  add  to  every  100  c.c.  of  acid  I0°  '*  - — ""  *  *'  = 

ri  —   i 

60  c.c.  of  water.  500  c.c.  of  strong  acid  are  mixed  with  the 
quantity  of  water  required  to  reduce  its  specific  gravity  to  i  -i, 
and  the  mixture  is  brought  into  a  retort  connected  with  a  good 
condensing  arrangement,  and  boiled  until  nearly  one-half  the 
amount  has  distilled  over.  The  ebullition  may  be  rendered 
more  regular  by  throwing  a  few  scraps  of  clean  platinum  foil 
into  the  liquid.  The  residue  contains  about  20-24  per  cent, 
of  HC1.  1 80 -8  grams  of  such  acid,  when  diluted  to  a  litre, 
furnish  a  solution  which  is  approximately  normal. 


Standard  Hydrochloric  Acid.  131 

Since  this  acid  is  of  frequent  application,  the  student 
should  prepare  from  2  to  3  litres  of  its  solution.  To  deter- 
mine its  exact  strength,  50  c.c.  of  the  acid  solution  are  run 
into  the  -J-litre  flask,  and  diluted  to  the  contatning-raaxk  after 
shaking,  i  c.c.  approximately  equals  0*003646  gram  HC1. 
Call  this  solution  A;  25  c.c.  of  A  are  further  diluted  to  250 
c.c.  i  c.c.  =  '0003646  gram  HC1.  Call  this  solution  B. 

Weigh  out  exactly  1*0794  gram  of  pure  silver  into  a 
bottle  of  about  300  c.c.  capacity,  provided  with  a  well-fitting 
stopper,  and  dissolve  the  metal  in  pure  dilute  nitric  acid. 
The  solution  should  be  heated  on  the  water-bath,  and  the 
fumes  of  the  oxides  of  nitrogen  should  be  blown  out  of  the 
bottle  from  time  to  time.  When  the  silver  is  completely 
dissolved,  and  the  liquid  on  agitation  gives  no  trace  of  red 
fumes,  the  bottle  is  removed  from  the  bath  and  allowed  to 
cool.  A  100  c.c.  pipette  is  rinsed  with  a  small  quantity  of 
solution  A,  which  is  allowed  to  flow  away.  The  pipette  is 
filled  to  the  mark  with  the  solution  A,  and  emptied  into  the 
silver  solution.  The  stopper  is  inserted,  and  the  solution  is 
briskly  agitated  for  some  time  until  the  silver  chloride  settles 
out  completely  and  leaves  the  liquid  almost  clear.  If  the 
hydrochloric  acid  is  of  exact  strength,  that  is,  if  it  is  strictly 
normal,  and  if  1*0794  gram  of  pure  silver  has  been 
accurately  weighed  out,  we  ought  of  course  to  have  neither 
silver  nor  chlorine  in  excess  in  solution.  In  all  probability 
we  shall  have  one  or  other  of  the  bodies  in  excess.  To 
determine  which  of  the  two  remains  in  solution,  we  add  i  c.c. 
of  deci-normal  silver  solution  and  note  whether  a  further 
turbidity  ensues.  If  the  liquid  remains  clear,  the  silver  is 
in  all  probability  already  in  excess  :  if  it  becomes  turbid  it  is 
a  sign  that  the  chlorine  is  present  in  excess.  In  the  latter 
case  the  solution  is  again  vigorously  agitated  until  the 
liquid  is  once  more  clear.  We  will  assume,  by  way  of 
example,  that  the  addition  of  the  i  c.c.  of  deci-normal  silver 
solution  produced  a  turbidity.  Another  i  c.c.  of  silver  solu- 
tion is  added  to  the  liquid,  and  we  again  observe  whether  a 
turbidity  is  caused.  Let  us  suppose  that  the  liquid  now 


132  Quantitative  Chemical  Analysis. 

remains  clear :  it  is  evident  that  an  excess  of  silver  is  in 
solution.  The  total  amount  of  silver  in  the  bottle  is  there- 
fore 1*0794  +  ('010.794  x  2)  =  1*100988  gram.  If  we 
determine  the  amount  in  solution,  we  can  at  once  tell  how 
much  is  precipitated  as  chloride,  and  accordingly  calculate 
the  amount  of  HClin  the  100  c.c.  of  diluted  acid.  It  is 
evident  that  we  have  at  least  0*010794  gram  of  silver  in 
excess,  since  the  addition  of  the  last  i  c.c.  of  deci-normal 
silver  solution  failed  to  produce  a  turbidity.  We  now  add 
i  c.c.  of  solution  A,  and  shake  the  liquid  vigorously  until  it 
is  clear.  In  all,  we  have  added  101  c.c.  of  solution  A,  and 
1*100988  gram  silver.  We  now  add  i  c.c.  of  solution  B, 
and  note  whether  a  turbidity  is  produced ;  if  so,  we  again 
shake  the  liquid  vigorously  until  it  is  clear,  and  add  a 
second  i  c.c.  :  if  the  liquid  is  still  rendered  turbid,  we  again 
shake  briskly,  and  add  a  third  i  c.c.,  and  so  on  until  the 
addition  of  i  c.c.  of  solution  B  no  longer  produces  any 
change.  This  last  c.c.  is  not  counted,  since  it  shows  that 
the  HC1  is  once  more  in  slight  excess,  and  we  shall  be  nearer 
the  truth  if  we  assume  that  only  half  of  the  preceding  c.c.  is 
necessary  for  precipitation.  In  working  with  the  centi- 
normal  solution  (solution  B)  it  is  necessary  that  the  liquid 
above  the  precipitated  silver  chloride  be  perfectly  clear,  and 
that  we  wait  for  a  few  seconds  (say  \  minute)  before  we  con- 
clude that  no  further  turbidity  is  caused  by  the  addition  of 
i  c.c.  of  solution.  Let  us  suppose  that  we  found  it  necessary 
to  add  5  c.c.  of  solution  B  before  the  liquid  remained  clear  : 
3*5  c.c.  are  therefore  necessary  to  precipitate  the  silver  in 
solution  after  the  addition  of  the  i  c.c.  of  solution  A.  If 
the  directions  given  have  been  properly  followed,  we  may 
assume  without  sensible  error  that  i  c.c.  of  solution  B  con- 
tains 0*000365  gram  HC1 :  this  is  equal  to  '0010794  gram  Ag., 
and  '0010794x3*5  =  0-003778  gram  Ag.  101  c.c.  of  solu- 
tion A  are  equivalent  to  1-100988  —  '003778  =  1*09721 
gram  Ag.,  and  accordingly 

107-94   :   36-45  ::  1*09721    :   x 

x  =  0-370514. 


Standard  Sulphuric  A  cid.  133 

101  c.c.  of  solution  A  contain  0-370514  gram  HC1.  Ac- 
cordingly the  500  c.c.  would  contain  1*83423  gram  HC1. 
But  the  500  c.c.  of  A  are  equivalent  to  50  c.c.  of  the  original 
acid.  Accordingly  i  c.c.  of  the  original  acid  contains 
•036684  gram  HC1,  instead  of  "03645  gram,  the  quantity 
required  to  constitute  the  normal  acid.  Instead  of  diluting 
the  acid  to  bring  it  to  the  exact  strength,  it  is  better  to  ex- 
press the  difference  by  a  small  factor:  in  this  case  °3 — i-= 

•03645 

1-0064.  The  acid  is  accordingly  labelled  '  Standard  Hydro- 
chloric Acid,  i  c.c.  =  0*03645  x  1*0064  gram  HCl? 
i  c.c.  of  the  acid  is  equivalent  to  0*10794  x  1*0064  gram 
Ag.,  or  0-04004  x  1-0064  gram  NaHO. 

At  least  two  determinations  should  be  made  of  the 
strength  of  the  acid  before  it  is  used,  and  the  mean  result 
-should  be  taken  as  indicating  the  correct  value. 

Preparation  of  Normal  Sulphuric  Acid  Solution. — In 
certain  processes  the  use  of  standard  hydrochloric  acid  is 
inadmissible  ;  in  such  cases  we  may  generally  employ  a 
normal  solution  of  sulphuric  acid.  This  may  be  prepared 
by  diluting  about  60  c.c.  of  concentrated  and  pure  sulphuric 
acid  with  five  or  six  times  its  volume  of  water,  allowing  the 
mixture  to  cool,  and  making  it  up  to  2  litres.  If  the  sul- 
phuric acid  used  was  concentrated,  the  solution  will  now 
contain  rather  more  than  49  grams  H2SO4  per  litre.  To 
determine  its  exact  strength,  weigh  out  about  2  grams  of 
recently  heated  pure  sodium  carbonate  into  a  weighed 
platinum  basin,  dissolve  it  in  a  small  quantity  of  water, 
cover  the  solution  with  a  watch-glass,  draw  the  watch-glass 
aside  and  add  25  c.c.  of  the  acid.  Place  the  liquid  on  a  water- 
bath,  and  as  soon  as  the  evolution  of  gas  has  ceased,  remove 
the  cover,  rinse  its  under  surface  into  the  dish,  and  evaporate 
the  liquid  to  complete  dryness.  Heat  to  180°  in  the  air-bath 
until  the  weight  is  constant.  The  calculation  is  very  simple. 
SO4=96  has  displaced  CO3=  60.  The  increase  in  weight  of 
the  dish  is  proportional  to  the  amount  of  sulphuric  acid 


134  Quantitative  Chemical  Analysis. 

employed — so  long  of  course  as  there  is  excess  of  sodium 
carbonate  present.  The  amount  of  sulphuric  acid  x  in  the 
25  c.c.  is  thus  found  : — 

The  difference  between  the  equivalent  of  SO4  and  CO3, 
yiz.  36,  is  to  the  equivalent  of  SO4H2,  viz.  98,  as  the  differ- 
ence between  the  first  and  second  weighing  of  the  platinum 
basin  is  to  the  sulphuric  acid  present  in  the  25  c.c. 

Example. — Weighed  out  17210  gram  of  pure  dry  sodium 
carbonate,  added  25  c.c.  of  the  acid,  evaporated  to  com- 
plete dryness,  and  heated  in  the  air-bath.  The  difference 
between  the  weight  of  the  dish  +  carbonate,  and  dish  -f- 
mixed  sulphate  and  carbonate  was  0*465  gram  :  then 

36    I    98     \\     0*465    :    x 

,..        ,    1*266 
x  =  1*266  and  =     0*05064. 

25 

i  c.c.  of  the  acid  accordingly  contained  0*05064  gram 
H2S04. 

This  method  of  determining  the  exact  strength  of  the  acid 
solution  is  quite  as  accurate,  and  certainly  more  convenient, 
than  precipitating  the  sulphuric  acid   as   barium  sulphate. 
By  way  of  control  the  acid  solution  was  treated  with  barium 
chloride  and  the  precipitate  washed,  dried,  and  weighed. 
10  c.c.   of  the  solution  gave   1*2043  gram  BaSO4  as   the 
mean  of  three  concordant  experiments.     This  is  equal  to 
0*5063  gram  of  sulphuric   acid.       The   determination  by 
sodium  carbonate  gave  0*5064  gram.     The  determination 
of  the  strength  of  the  acids  employed  in  volumetric  analysis 
may,  in  many  cases,  be  accurately  and  expeditiously  made 
by  means  of  metallic  sodium.     About  0*5  gram,  of  clean 
freshly-cut  sodium  is  placed  in  a  short  wide-mouthed  test-tube, 
previously  weighed,  together  with  its  well-fitting  cork ;  the 
tube  containing  the  sodium  is  corked  and  again  weighed  ;  the 
metal  is  thrown  upon  about  15  c.c.  of  cold  water  contained 
in  a  porcelain  basin,  which  is  then  quickly  covered  with  a 
large  watch-glass.     When  the  action  is  at  an  end,  the  glass 
is  raised,  a  few  drops  of  litmus  solution  (vide  infra)  added, 


Standard  Soda  Solution.  135 

and  the  acid  is  run  in  from  a  burette  until  the  red  tint  is 
permanent. 

The  determination  of  the  strength  should  be  repeated  once 
or  twice,  and  the  mean  result  taken  as  expressing  the  true 
value  of  the  acid.  If  the  solution  is  approximately  normal 
it  is  better  not  to  dilute  it,  but  to  calculate  the  factor  required 
to  bring  its  strength  to  the  normal  value.  Thus  the  factor  of 
the  above-mentioned  solution  would  be  -f^  =  1*0334. 
It  would  be  labelled  therefore  '  Normal  Sulphuric  Acid,  i 
c.c.='Q^gram  x  1-0334  SO4H2.' 

If  the  acid  is  much  above  the  normal  value  it  will  be 
more  convenient  to  dilute  it  so  as  to  make  it  as  nearly  as 
possible  of  the  proper  strength.  Thus,  supposing  that  we 
had  found  that  i  c.c.  contained  0-055  gram  H2SO4,  1000  c.c. 
would  contain  55  grams.  Consequently,  according  to  the 
proportion 

49    :    1000    M    55    :    x.   x  —  1123. 

we  must  add  123  c.c.  of  water  to  every  1000  c.c.  of  the  acid 
solution.  This  may  be  best  eifected  by  rilling  the  litre  flask 
up  to  the  wntazmng-maxk  with  the  acid  solution,  and  emptying 
it  into  the  dry  and  clean  bottle  in  which  it  is  to  be  preserved. 
Now  pour  into  the  litre  flask  123  c.c.  of  water  (by  means  of 
the  100  c.c.  pipette  and  the  burette),  shake  the  liquid  about 
in  the  flask  and  pour  it  into  the  bottle,  and  shake  the 
mixture.  Again  pour  about  half  the  acid  liquid  back  into 
the  litre  flask,  shake  and  transfer  it  once  more  to  the  bottle. 

Preparation  of  Normal  Caustic  Soda  Solution. — Dissolve 
from  42  to  45  grams  of  sodium  hydrate  in  800  c.c.  of  water, 
and  titrate  the  solution  by  normal  acid  and  litmus  tincture. 
The  alkaline  solution  is  then  diluted  until  it  possesses  the 
normal  strength. 

The  caustic  soda  solution  may  also  be  obtained  by  dis- 
solving about  150  grams  of  pure  dry  sodium  carbonate  in  3 
litres  of  water,  boiling  the  solution  in  a  clean  iron  vessel,  and 
adding,  little  by  little,  80  grams  of  freshly-burnt  lime  made 


136 


Qtiantitative  Chemical  Analysis. 


into  a  cream  with  water.*  The  mixture  must  be  boiled  until 
a  small  quantity  of  the  clear  solution  no  longer  effervesces 
on  the  addition  of  an  acid  in  excess.  The  iron  vessel  is  then 
closely  covered,  and  after  standing  12  or  14  hours  the  clear 
alkaline  solution  is  drawn  off  by  the  aid  of  a  syphon. 

Preparation  of  Litmus  Solution. — 5  or  6  grams  of  coarsely- 
powdered  litmus  are  digested  with  about  200  c.c.  of  distilled 
water  for  a  few  hours.  The  clear  solution  is  decanted  from 
the  sediment,  and  very  dilute  nitric  acid  added,  drop  by  drop, 
until  the  colour  is  changed  to  violet.  The  solution  must  be 
neither  red  nor  blue,  but  between  the  two  in  colour  ;  when 
properly  neutralised  less  than  ^  c.c.  of  the  standard  acid 
should  distinctly  redden  the  solution  of  i  c.c.  in  100  c.c.  of 
water  ;  on  the  other  hand  the  same  amount  of  standard  alkali 
should  render  the  colour  decidedly  blue.  The  solution  should 
be  kept  in  a  wide-mouthed  bottle,  the  cork  of  which  is  so  cut 
FlG  44  that  the  air  has  ready  access 

to  the  interior  of  the  bottle, 
otherwise  the  liquid  quickly 
loses  its  colour.  Through  the 
cork  is  fitted  a  short  tube  on 
which  is  a  mark  ;  this  tube 
serves  to  deliver  a  determinate 
volume  of  the  litmus  solution. 
Determination  of  the  Strength 
of  the  Caustic  Soda  Solution. — 
25  c.c.  of  the  standard  sul- 
phuric acid  solution  are  poured 
into  a  porcelain  basin,  mixed 
with  a  measure  of  litmus  solu- 
tion, and  the  alkaline  liquid  is 
added  drop  by  drop  from  a 
burette  until  the  colour  is  just 
turned  to  blue.  Repeat  the 

*  If  this  quantity  of  pure  dry  sodium  carbonate  is  not  at  hand,  250 
grams  of  the  bicarbonate  are  heated  to  dull  redness  in  a  platinum  basin, 
in  small  portions  at  a  time,  for  ten  or  fifteen  minutes,  to  expel  the 
carbon  dioxide.  The  salt  is  then  treated  as  above. 


Soda- Ash.  137 

determination,  take  the  mean  of  the  two  observations,  and 
dilute  the  alkaline  solution  until  it  corresponds  volume  for 
volume  with  the  standard  acid.  Thus,  supposing  you  have 
found  that  25  c.c.  of  acid  required  22  c.c.  of  soda  for  neu- 
tralisation, you  will  require  to  add  3  c.c.  of  water  to  every 
22  c.c.  of  lye,  or  each  litre  of  alkaline  liquid  will  require  the 
addition  of  136  c.c.  of  water. 

The  diluted  liquid  should  be  poured  into  a  large  bottle 
fitted  with  a  syphon  and  wide  tube  as  shown  in  Fig.  44.  The 
wide  tube  is  filled  with  soda-lime  in  small  pieces  to  prevent 
the  entrance  of  carbon  dioxide.  A  thin  layer  of  refined 
petroleum  or  paraffin  oil  poured  on  the  surface  of  the  liquid 
greatly  tends  to  the  preservation  of  its  strength.  The  exact 
strength  of  the  diluted  liquid  should  then  be  determined  by 
neutralising  varying  qualities,  say  25  c.c.,  30  c.c.,  and  50  c.c., 
of  standard  acid  in  the  manner  above  described.  The  mean 
result  of  the  observations  should  be  taken  as  the  true  value. 

IV.    VALUATION  OF  SODA-ASH. 

Soda-ash  is  a  crude  sodium  carbonate;  its  value  depends  on 
the  amount  of  available  sodium  carbonate  which  it  contains. 
Its  impurities,  in  addition  to  moisture,  mainly  consist  of 
sodium  hydrate,  sulphate,  chloride,  silicate,  and  aluminate. 
It  also  not  unfrequently  contains  sodium  sulphide,  sulphite, 
and  thiosulphate. 

Weigh  out  about  10  grams  of  the  powdered  sample  into  a 
weighed  platinum  crucible,  and  heat  gently  for  twenty  or 
thirty  minutes  over  the  lamp ;  place  the  crucible  in  the 
dessicator,  and  weigh  when  cold.  The  loss  of  weight  gives 
the  amount  of  moisture  contained  in  the  sample.  Transfer 
the  weighed  salt  to  a  beaker,  wash  out  the  crucible,  and 
dissolve  the  salt  in  a  small  quantity  of  water,  filter  (if  neces- 
sary) into  the  J-litre  flask,  wash  the  filter  thoroughly,  and 
dilute  to  the  containing-mark,  and  shake.  Take  out  50 
c.c.  of  the  solution,  corresponding  to  i  gram  of  soda-ash, 
pour  the  liquid  into  a  flask,  and  add  25  c.c.  of  standard 
sulphuric  acid,  and  boil  the  solution  for  some  time  until  the 


138  Quantitative  Chemical  Analysis. 

carbonic  acid  is  expelled.  Add  a  measure  of  litmus  solu- 
tion, and  standard  soda-soluti©n  from  a  burette,  drop  by 
drop,  until  the  blue  colour  of  the  litmus  is  restored. 

Example. — 10-025  grams  of  soda-ash  were  heated,  dis- 
solved in  water,  and  the  clear  solution  made  up  to  500  c.c. 
50  c.c.  (corresponding  to  1*0025  gram  soda-ash)  were  trans- 
ferred to  a  flask,  25  c.c.  of  standard  sulphuric  acid 
(i  c.c.  =  '049  gram  SO4H2  x  1*0204)  were  added,  the  liquid 
was  boiled,  mixed  with  a  measure  of  litmus  solution,  and 
standard  soda  added,  drop  by  drop,  until  the  blue  colour 
was  restored.  10  c.c.  of  soda  solution=9*8  c.c.  of  standard 
sulphuric  acid.  9*2  c.c.  of  the  alkaline  liquid  were  needed. 

10    \    9*2  \\   9*8    I    9*0 

Accordingly  25—9*0=16*0  c.c.  of  standard  acid  have  been 
used  to  decompose  the  1*0025  g1"3-111  of  soda-ash.  But 
i  c.c.  of  acid  contains  0*049x1*0204  gram  of  sulphuric 
acid  ;  this  corresponds  to  0*053  x  1*0204  of  sodium  carbo- 
nate. The  amount  of  sodium  carbonate  in  the  1*0025 
gram  of  original  soda-ash  is  therefore  -053  x  1*0204  x  16*0 
=0*865  £ram>  °r  m  IO°  parts  1*0025  •  Io°  ••  0*865 
=86 '3  per  cent. 

The  value  of  pearl-ash  may  be  determined  in  exactly  the 
same  manner.  It  must  not  be  forgotten  that  dried  potas- 
sium carbonate  is  very  hygroscopic  ;  due  expedition  must, 
therefore,  be  employed  in  weighing  this  body.  The  crucible 
should  be  closely  covered  during  the  operation.  The 
\  equivalent  of  potassium  carbonate  is  69*1  ;  accordingly, 
the  factor  -0691  is  used  instead  of  -053  in  the  calculation. 

V.   ESTIMATION  OF  ALKALINE  HYDRATE  IN  PRESENCE  OF 
CARBONATE. 

The  crude  carbonates  of  soda  and  potash  not  unfrequently 
contain  notable  quantities  of  hydroxide.  The  alkaline  lyes 
used  by  paper,  soap,  and  starch  manufacturers  also  consist 
of  mixtures  of  carbonated  and  caustic  alkalies. 

To  estimate  the  proportion  of  the  two  constituents,  a  deft- 


Sodium  and  Potassium  Carbonates.  139 

nite  quantity  of  the  salt  or  solution,  say  15  grams,  or  50  c.c., 
is  dissolved  in  water,  and  diluted  to  250  c.c.  The  total 
amount  of  alkali  is  then  determined,  say  in  50  e.c.,  in  the 
manner  described  on  p.  137,  viz.,  by  standard  acid,  litmus, 
and  soda  solutions.  Take  out  100  c.c.  of  the  alkaline  solu- 
tion, pour  it  into  a  300  c.c.  flask,  dilute  with  a  little  water, 
heat,  and  add  solution  of  barium  chloride  so  long  as  a  pre- 
cipitate is  formed.  The  reactions  which  occur  in  the  case 
of  sodium  carbonate  and  hydrate  are : 

2NaHO  +   BaCl2  =  2NaCl  +   BaH2O2, 
and        Na2CO3  +  BaCl2  =  2NaCl  +  BaCO3. 

Fill  up  the  flask  to  the  containing-mark,  shake,  and  allow 
the  precipitate  to  settle.  Withdraw  100  c.c.  of  the  clear 
liquid,  pour  it  into  a  porcelain  basin,  add  an  excess,  but  in 
measured  quantity,  of  standard  hydrochloric  acid,  a  measure 
of  litmus  solution,  and  then  determine  the  excess  of  acid 
added  by  means  of  the  soda-solution.  Multiply  the  amount 
of  hydroxide  found  by  7-5  ;  this  gives  the  amount  of  caustic 
alkali  in  the  weight  of  the  substance  originally  taken. 

VI.  ESTIMATION  OF  SODIUM  CARBONATE  IN  PRESENCE  OF 
POTASSIUM  CARBONATE. 

Sodium  carbonate,  on  account  of  its  cheapness,  is  some- 
times employed  to  adulterate  pearl-ash.  The  quantity  of  the 
admixed  sodium  salt  may  be  estimated  in  the  following 
manner.  About  5  grams  of  the  mixture  are  gently  heated 
in  a  weighed  platinum  crucible  for  fifteen  or  twenty  minutes; 
the  loss  on  weighing  gives  the  amount  of  moisture  present. 
The  dried  mass  is  dissolved  in  a  small  quantity  of  water, 
and  filtered,  if  necessary ;  acetic  acid  is  added  in  slight 
excess,  the  liquid  is  heated  to  expel  carbonic  acid,  and 
mixed  with  a  dilute  solution  of  lead  acetate  so  long  as  a 
precipitate  of  lead  sulphate  is  formed.  The  liquid  is 
filtered,  and  the  excess  of  lead  removed  by  a  stream  of 
sulphuretted  hydrogen;  the  solution  is  again  filtered  into  a 
250  c.c.  flask,  and  diluted  to  the  containing-mark.  50  c.c. 


140 


Quantitative  Chemical  Analysis. 


of  the  liquid  are  then  evaporated  to  dryness,  with  about  10 
c.c.  of  dilute  hydrochloric  acid  (IT  sp.  gr.)  in  a  weighed  pla- 
tinum dish.  The  residual  chlorides  are  dried  and  weighed  ; 
the  relative  proportion  of  the  potash  and  soda  is  then  deter- 
mined by  means  of  standard  silver  and  potassium  chromate, 
in  the  manner  described  on  p.  128. 

VII.     DETERMINATION  OF  AMMONIA. 

The  quantity  of  free  ammonia  in  solution  may  be  deter- 
mined, as  in  the  case  of  caustic  soda  or  potash,  by  means  of 
standard  acid  and  litmus  solutions.  A  definite  quantity  of 
the  solution,  say  10  c.c.,  is  transferred  to  a  small  tared  flask, 
and  weighed  :  its  absolute  weight  and  specific  gravity  are 
thus  determined  in  a  single  operation.  If  the  10  c.c.  weighed 
9*0  grams,  its  specific  gravity  would  of  course  be  0-9000, 
water  being  i.  The  weighed  quantity  of  the  ammonia  is 
then  diluted  with  6  or  8  times  its  bulk  of  water  and  titrated 
directly  in  the  ordinary  manner  by  the  standard  acid. 

The  operation  of  taking  the  specific  gravity  and  weighing 
out  the  ammonia  solution  may  be  rendered  more  accurate, 

especially  if  the  solution  is 
very  strong,  by  the  aid  of  the 
little  apparatus  seen  in  Fig.  45. 
In  the  hole  of  the  caoutchouc 
ball  a  is  inserted  a  brass  tube 
running  through  and  fixed  into 
the  plate  b.  The  end  of  the  tube 
is  closed,  its  upper  edge  is 
filed  through,  and  over  it  is 
slipped  a  piece  of  tightly-fitting 
caoutchouc  tube,  in  which, 
immediately  over  the  orifice  in 
the  brass  tube,  is  a  hole  pierced 
by  a  pin.  Through  the  hole  is 
inserted  the  end  of  the  appara- 
tus cct  which  is  further  supported  by  the  holder.  The 
caoutchouc  ball  maybe  compressed  by  the  plate  d,  moveable 


FIG.  45. 


Ammonia.  141 

along  the  rods,  by  the  aid  of  the  milled-head  screw  s,  On 
compressing  the  ball,  the  air  makes  its  escape  through  the 
end  e  of  the  apparatus,  and  if,  after  the  expulsion  of  the  air, 
the  end  ^be  placed  beneath  the  surface  of  a  liquid,  the  liquid 
will  be  driven  into  the  apparatus  in  proportion  as  the  screw 
is  reversed.  If  the  apparatus  has  the  arrangement  seen  in  the 
figure,  it  is  evident  that  it  may  be  withdrawn  from  the  caout- 
chouc tube  without  any  of  the  liquid  flowing  out.  If  the 
capacity  of  the  bulb  up  to  a  certain  mark,  say  at  m,  be  accu- 
rately estimated,  by  determining  the  weight  of  water  it 
contains  up  to  that  mark,  we  can  readily  determine  the 
specific  gravity  of  any  liquid  introduced  into  it  by  simply 
weighing  the  apparatus  filled  with  the  liquid.  By  reinserting 
the  end  into  the  hole  in  the  caoutchouc  tube  and  compressing 
the  ball  we  can  deliver  any  required  quantity  of  the  liquid. 
On  again  weighing  the  apparatus,  its  loss  of  weight  im- 
mediately gives  us  the  amount  of  liquid  delivered.  The 
apparatus  may  also  be  used  for  weighing  out  and  determining 
the  specific  gravities  of  strong  acids,  fuming  liquids,  &c. 

Ammonia  in  combination  may  be  determined  by  expelling 
it  by  means  of  caustic  soda  or  lime,  collecting  the  evolved 
ammonia  in  an  excess  of  standard  acid,  and  determining  the 
excess  of  acid  by  soda- solution.  The  ammoniacal  compound 
(say  ammonium  chloride)  is  weighed  out  into  the  retort  a, 
Fig.  32,  and  the  ammonia  collected  in  excess  of  standard 
acid  j  the  amount  of  the  residual  acid  is  then  determined  by 
soda-solution. 

The  ammonia  contained  in  many  commercial  salts,  in 
ammoniacal  gas-liquor,  &c.,  may  be  determined  by  this 
method.  In  estimating  the  ammonia  in  guano,  magnesia 
must  be  employed  instead  of  lime  or  soda,  otherwise  the 
nitrogenous  organic  matter  present  will  be  partially  decom- 
posed with  evolution  of  ammonia. 


142  Quantitative  Chemical  Analysis. 

ACIDIMETRY. 

The  principles  involved  in  the  estimation  of  the  strength 
of  acid  solutions  are  identical  with  those  we  have  indicated 
under  Alkalimetry.  The  value  of  the  strong  acids,  such  as 
hydrochloric,  nitric,  and  sulphuric  acids,  is  frequently  deduced 
from  their  specific  gravities,  and  comprehensive  tables  have 
been  calculated  showing  the  percentage  amount  of  the 
various  acids  in  solutions  of  different  densities  (see  Appen- 
dix). Occasionally  it  is  necessary  to  control  the  indications 
of  the  hydrometer,  or  specific-gravity  bottle,  by  titrating  the 
acid  solution.  The  apparatus  described  on  p.  140  (Fig.  45) 
may  be  conveniently  employed  to  determine  both  the 
specific  gravity  of  the  liquid  and  the  weight  taken  for  an- 
alysis. The  determination  of  the  strength  of  nitric,  hydro- 
chloric, and  sulphuric  acids  by  caustic  soda  and  litmus  solu- 
tion presents  no  difficulties  :  the  method  will  be  evident 
from  the  foregoing  descriptions  under  Alkalimetry. 

VIII.     DETERMINATION  OF  THE  STRENGTH  OF  ACETIC, 
ACID,  PYROLIGNEOUS  ACID,  VINEGAR. 

The  estimation  of  the  strength  of  this  acid  in  its  various 
forms  cannot  be  made  with  very  great  accuracy  by  direct 
titration  with  caustic  soda  solution,  since  sodium  acetate 
possesses  a  feeble  alkaline  reaction,  which  interferes  with  the 
correct  determination  of  the  final  point  The  method  most 
generally  applicable  consists  in  adding  to  a  known  quantity 
of  the  acid,  a  weighed  quantity  (in  excess)  of  finely-powdered 
marble,  heating  the  liquid  to  boiling,  filtering,  washing  the 
residual  calcium  carbonate  with  hot  water,  dissolving  it  in  a 
slight  excess  of  normal  hydrochloric  acid,  and  titrating  with 
caustic  soda  and  litmus  solution.  This  method  is  par- 
ticularly useful  in  testing  brown  pyroligneous  acid  or  highly 
coloured  vinegars. 

[Note. — Instead  of  litmus  solution,  a  dilute  tincture  of 
cochineal  may  be  employed.  It  may  be  prepared  by  digesting 
2  or  3  grams  of  powdered  cochineal  in  200  c.c.  of  a  mixture 


Carbon  Dioxide. 


143 


of  i  part  of  alcohol  and  4  of  water.  It  forms  an  orange 
solution  which  is  turned  violet  by  alkalies  :  the  colour  is 
almost  unaffected  by  carbonic  acid.] 

IX.     DETERMINATION  OF  COMBINED  CARBON  DIOXIDE. 

The  amount  of  carbon  dioxide  in  soluble  carbonates  may 
be  readily  determined  by  decomposing  them  with  a  solution 
of  calcium  chloride,  throwing  the  precipitated  calcium  car- 
bonate on  to  a  filter,  washing  thoroughly  with  hot  water, 
dissolving  in  an  excess  of  standard  hydrochloric  acid,  and 
determining  the  excess  of  acid  by  standard  soda  solution  in 
the  usual  manner. 

The  acid  carbonates  (bicarbonates)  require  the  addition  of 
ammonia,  with  the  calcium  chloride. 

The  carbon  dioxide  in  insoluble  carbonates,  as  in  cala- 
mine,  ferrous  carbonate,  white  lead,  mortar,  cements,  &c., 
is  determined  by  expelling  the  gas  by  the  action  of  hydro- 
chloric acid,  absorbing  it  by 
ammonia,  and  precipitating  by 
the  addition  of  calcium  chlo- 
ride. The  calcium  carbonate  is 
further  treated  in  the  manner 
above  described. 

The  decomposition  may 
conveniently  be  effected  in  the 
apparatus  seen  in  fig.  46. 
The  flask  A,  of  about  150  c.c. 
capacity,  contains  the  weighed 
quantity  of  carbonate,  together 
with  about  10  c.c.  of  water:  it 
is  fitted  with  a  caoutchouc 
cork,  in  which  are  inserted  the 
bent  tube  a  and  the  pipette- 
shaped  tube  £,  filled  with 
moderately-concentrated  hy- 
drochloric acid.  The  flow  of 


FIG.  46. 


144  Quantitative  Chemical  Analysis. 

the  acid  into  the  flask  may  be  easily  regulated  by  the  clip. 
The  flask  B  contains  TO  or  15  c.  c.  of  ammonia-water:  the 
tube  must  not  dip  into  the  liquid.  The  wider  tube  c  is 
partially  filled  with  broken  glass  moistened  with  ammonia- 
water.  Care  should  be  taken  that  the  ammonia  is  free 
from  carbonic  acid:  its  purity  may  be  tested  by  adding  to  it 
a  few  drops  of  calcium  chloride  solution  :  if  free  from  car- 
bonate it  will  remain  perfectly  clear. 

To  make  a  determination,  warm  the  flask  B,  so  as  to  fill  it 
with  an  atmosphere  of  ammonia,  and  then  cautiously  allow 
a  few  drops  of  hydrochloric  acid  to  fall  on  to  the  weighed 
quantity  of  carbonate  in  A.  As  soon  as  the  whole  of  the 
carbonate  is  decomposed,  heat  the  liquid  in  A  to  boiling,  so 
as  to  expel  the  last  trace  of  carbon  dioxide,  and  keep  it 
boiling  for  a  few  minutes.  Wash  the  bent  tube  a  and  also 
the  glass  in  t,  add  calcium  chloride  solution  to  the  ammo- 
niacal  liquid,  boil  for  some  time,  filter,  wash  thoroughly,  and 
titrate  the  calcium  carbonate  in  the  manner  already  described. 
The  operation  of  filtering  and  washing  should  be  done  as 
expeditiously  as  possible,  since  the  ammoniacal  liquid 
absorbs  atmospheric  carbon  dioxide. 

X.  ESTIMATION  OF  CARBONIC  ACID  IN  NATURAL  WATERS. 

The  amount  of  carbonic  acid  in  spring,  river,  or  mineral 
water  may  be  accurately  estimated  in  the  following  manner. 
100  c.c.  of  the  water  to  be  examined  are  transferred  to  a  dry 
flask,  together  with  3  c.c.  of  a  strong  neutral  solution  of 
calcium  chloride,  and  2  c.c.  of  a  saturated  solution  of 
ammonium  chloride.  50  c.c.  of  lime-water,  the  strength  of 
which  is  accurately  known,  are  then  added,  the  flask  is 
closed  by  a  caoutchouc  cork,  and  its  contents,  amounting  to 
155  c.c.,  agitated.  In  about  twelve  hours  the  whole  of  the 
calcium  carbonate  will  have  separated  out,  and  the  liquid 
will  be  perfectly  clear.  50  c.c.  of  the  clear  liquid  are  with- 
drawn, and  the  residual  amount  of  lime  determined  by 
deci-normal  hydrochloric  acid. 


Carbon  Dioxide.  145 

A  solution  of  oxalic  acid  may  be  conveniently  substituted 
for  the  hydrochloric  acid  ;  it  should  be  made  of  such  a 
strength  that  i  c.c.  is  equivalent  to  i  milligram  of  carbon 
dioxide.  This  solution  may  be  obtained  by  dissolving 
2-8636  grams  of  pure  dry  crystallised  oxalic  acid  in  water, 
and  diluting  to  i  litre.  The  lime-water  should  be  of  such 
strength  that  25  c.c.  are  equal  to  23  or  24  c.c.  of  acid.  The 
final  point  of  the  reaction  may  be  determined  by  the  aid  of 
a  drop  of  tincture  of  pure  rosolic  acid,  which  gives  a 
splendid  red  colour  in  presence  of  the  alkaline  earth,  which 
disappears  on  neutralising  with  an  acid.  Instead  of  rosolic 
acid,  turmeric  paper  may  be  used.  Swedish  paper,  in  strips, 
is  immersed  in  tincture  of  turmeric,  and  dried.  A  drop  of 
the  liquid  brought  upon  the  paper  gives  a  reddish-brown 
stain,  so  long  as  the  least  trace  of  the  alkaline  earth  remains 
in  the  free  state. 

Example. — 100  c.c.  of  the  water  of  the  Irish  Channel 
were  mixed  with  3  c.c.  of  calcium  chloride,  and  2  c.c.  of 
ammonium  chloride,  together  with  50  c.c.  of  lime-water. 
50  c.c.  of  lime-water =46 '4  c.c.  of  oxalic  acid  ; 

of  which  i  c.c.=i  milligram  CO2. 
After  standing  fifteen  hours — 

50  c.c.  of  solution  required  13*3  c.c.  of  standard  acid  for 
neutralisation. 

Therefore — 

46-4-(i3'3    x    3-1)     =     5-2. 
100  c.c.  of  the  sea-water  contain  5-2  milligrams  carbon  dioxide. 

XL    ESTIMATION  OF  CARBON  DIOXIDE  IN  ARTIFICIALLY 
AERATED  WATERS. 

The  determination  of  the  total  quantity  of  carbon  dioxide 
in  artificial  mineral  waters, — seltzer, — soda-water,  &c. — may 
be  readily  effected  in  the  following  manner.  A  narrow  brass 
cork-borer  is  pierced  with  two  or  three  small  holes,  about 
4  or  5  centimetres  from  the  edge.  A  piece  of  caoutchouc 
tubing  is  slipped  over  the  upper  end  of  the  borer,  and  the 

L 


146 


Quantitative  Chemical  Analysis. 


FIG.  47 


other  end  is  connected  with  the  bent  tube  a  of  the  flask  B3 
fig.  47.  The  flask  contains  about  25  or  30  c.c.  of  mode- 
rately strong  ammonia  ;  the  end  of  the  glass  tube  should  dip 
beneath  the  surface  of  the  liquid.  The  sides  and  edge  of 
the  brass  tube  are  rubbed  with  a  little  paraffin,  and  it  is  then 
screwed  through  the  cork  of  the  bottle  of  the  aerated  water 

by  holding  the  tube  stationary, 
and  turning  the  bottle  round, 
until  the  holes  make  their  ap- 
pearance below  the  cork.  The 
gas  is  immediately  liberated  j  it 
makes  its  escape  through  the 
holes  in  the  borer,  and  is  absorbed 
by  the  ammonia-water.  The 
greater  portion  of  the  residual 
gas  is  expelled  by  shaking  the 
water,  and  gently  heating  it  by 
surrounding  it  with  warm  water. 
As  soon  as  no  more  gas  is 

evolved,  the  cork  of  the  soda-water  bottle  is  withdrawn, 
and  the  liquid  added  to  that  contained  in  the  flask,  the 
tubes  are  washed,  calcium  chloride  is  added,  the  liquid 
is  boiled  for  some  time,  and  the  amount  of  calcium  carbo- 
nate determined  in  the  ordinary  manner.  The  quantity  of 
combined  carbon  dioxide  in  the  water  may  be  estimated  by 
evaporating  a  second  portion  to  dryness,  gently  igniting  the 
residue,  and  titrating  with  standard  acid  and  soda. 

XII.     DETERMINATION  OF  COMBINED  ACIDS  IN  SALTS. 

If  we  add  caustic  soda  to  a  boiling  solution  of  copper 
sulphate,  cupric  oxide  is  precipitated,  and  sodium  sulphate 
remains  in  solution  :  — 


CuS0 


H2O. 


4         2NaHO     =     CuO   +  Na2SO4 

.If,  therefore,  we  mix  a  measured  quantity  (in  excess)  of 
standard  soda  solution  with  a  solution  of  copper  sulphate, 


Oxidation  and  Reduction.  147 

and  boil,  the  amount  of  residual  alkali  indicates  the  quantity 
of  the  acid  contained  in  the  salt. 

Weigh  out  about  2  grams  of  copper  sulphate  into  a 
250  c.c.  flask,  dissolve  in  100  c.c.  of  water,  boil,  and  add 
excess  of  standard  soda  solution,  allow  to  cool,  and  dilute 
with  water  to  the  containing-mark,  cork  the  flask,  and  allow 
the  precipitate  to  settle.  Take  out  an  aliquot  portion  of  the 
clear  supernatant  liquid,  and  determine  the  amount  of  resi- 
dual alkali  in  solution.  The  method  of  calculating  the  result 
needs  no  explanation. 

Many  other  substances,  precipitable  by  sodium  hydroxide 
or  sodium  carbonate,  admit  of  estimation  by  this  method. 
Thus  we  may  determine  the  amount  of  acid  present  in 

Silver  salts,  with  caustic  soda. 
Mercury  salts,  with  caustic  soda. 
Bismuth  salts,  with  sodium  carbonate. 
Lead,  nickel,  cobalt,  zinc,  aluminium,  manganese,  alkaline 
earths,  and  magnesia,  with  sodium  carbonate. 

In  certain  cases  the  acid,  as  in  copper  sulphate,  or  mer- 
curic chloride,  may  be  liberated  by  treating  a  boiling  solution 
of  the  salt  with  sulphuretted  hydrogen,  filtering,  and  deter- 
mining the  amount  of  the  free  acid  by  caustic  soda  and 
litmus  solutions. 


ANALYSIS  BY  OXIDATION  AND  REDUCTION. 

We  have  already  explained  the  main  principle  of  this 
special  form  of  volumetric  analysis  ;  we  have  shown,  for 
example,  how  the  amount  of  iron  in  solution  may  be  esti- 
mated by  determining  the  quantity  of  oxygen  required  to 
convert  it  from  the  state  of  ferrous  to  that  of  ferric  oxide. 
A  large  number  of  other  substances  may  be  estimated  by 
the  aid  of  a  solution  of  a  substance  which,  like  potassium  per- 
manganate, readily  parts  with  a  portion  of  its  oxygen.  In 

L  2 


148  Quantitative  Chemical  Analysis. 

all  these  cases  the  amount  of  oxygen  given  up  is  taken  as  an 
index  of  the  quantity  of  the  substance  to  be  determined, 

Of  the  many  oxidising  agents  which  are  known,  the  most 
generally  applicable  are  (i)  potassium  permanganate  (per- 
manganic acid),  and  (2)  iodine. 


A,     ESTIMATIONS  BY   MEANS   OF   POTASSIUM 
PERMANGANATE. 

Preparation  of  Potassium  Permanganate  Solution. — About 
5  grams  of  pure  crystallised  potassium  permanganate  are  dis- 
solved in  a  small  quantity  of  water,  and  the  solution  is 
diluted  to  i  litre.  It  must  be  contained  in  a  glass-stoppered 
bottle,  which  should  be  kept  in  a  cool,  dark  place,  when  not 
in  use.  The  solution  thus  preserved  may  be  kept  for  a  long 
time  without  experiencing  much  alteration. 

XIII.     DETERMINATION  OF  THE  STRENGTH  OF  THE 
PERMANGANATE  SOLUTION. 

It  is  absolutely  necessary  to  determine  the  power  of  the 
permanganate  solution  by  direct  experiment  before  using  it; 
we  cannot  calculate  its  strength,  i.e.  the  amount  of  oxygen 
that  it  is  capable  of  furnishing,  from  the  weight  of  the  salt 
dissolved,  on  account  of  its  instability.  The  most  accurate 
method  of  estimating  the  strength  of  the  solution  consists  in 
determining  the  amount  required  to  transform  a  known 
weight  of  iron  from  the  condition  of  ferrous  oxide  to  that  of 
ferric  oxide. 

About  i  gram  of  fine  iron-wire  (piano  wire),  perfectly  free 
from  rust,  is  accurately  weighed  out  into  a  ^-litre  flask,  and 
dissolved  in  about  100  c.c.  of  dilute  sulphuric  acid  (i  pt.  of 
acid  to  6  of  water).  A  small  quantity  of  sodium  carbonate 
is  thrown  into  the  liquid  at  the  same  time  in  order  that  the 
air  within  the  flask  may  be  displaced  by  carbon  dioxide. 
The  flask  is  fitted  with  a  cork  and  bent  tube  furnished  with 


Use  of  Potassium  Permanganate.  149 

a  clip  ;  the  end  of  the  tube  dips  beneath  the  surface  of  about 
25  c.c.  of  water  contained  in  a  small  flask  (fig.  48). 
Whilst  the  iron  is  dissolving,  the  clip  is  kept  open  by 
slipping  it  over  the  glass  tube.  The  solution  of  the  iron 
may  be  accelerated  by  a  gentle  heat ;  the  liquid  is  gradually 
caused  to  boil,  and  maintained  in  brisk  ebullition  for  a 
minute  or  two,  so  as  to  expel  the  mixture  of  carbon  dioxide 
and  hydrogen,  and  the  caoutchouc  tube  is  immediately 
closed  and  the  lamp  removed.  In  a  minute  or  so  the  clip 
is  again  opened,  when  the  water  from  the  little  flask  is  driven 
over  into  the  solution  of  iron :  in  proportion  as  it  passes 
over,  boiling  water  is  poured  into  the  small  flask  until  the 

FIG.  48 


larger  one  is  nearly  filled.  The  caoutchouc  tube  is  once 
more  closed,  the  flask  and  its  contents  allowed  to  cool 
perfectly,  and  the  volume  of  the  liquid  made  up  to  the  con- 
taining-mark,  the  stopper  of  the  flask  is  inserted,  and  the 
liquid  thoroughly  mixed  by  shaking.  Whilst  the  solution  is 
cooling,  fill  a  Gay-Lussac's  or  a  Mohr's  burette  fitted 


15°  Quantitative  CJiemical  Analysis. 

with  a  glass  stop-cock  (the  permanganate  solution  gradually 
attacks  caoutchouc),  previously  rinsed  out  with  a  little 
of  the  permanganate  solution,  read  off  the  level  of  the  per- 
manganate solution,  take  out  50  c.c.  of  the  iron  solution, 
and  pour  it  into  about  200  c.c.  of  water  contained  in  a 
beaker,  standing  on  a  sheet  of  white  paper.  Add  the  per- 
manganate drop  by  drop  to  the  liquid,  with  constant  stirring, 
until  the  pink  colour  of  the  solution  is  permanent.  The 
permanganate  is  at  first  decomposed  with  great  rapidity  :  as 
the  iron  becomes  oxidised  the  colour  disappears  more  slowly; 
the  rapidity  of  the  change  indicates  the  progress  of  the 
oxidation.  The  operation  of  standardising  should  be  - 
repeated  once  or  twice  on  successive  portions  of  50  c.c.  of 
iron  solution,  and  the  mean  of  the  observations  taken  as 
representing  the  true  value  of  the  permanganate  solution. 

ioFeSO4  +   2KMnO4  +  8SO4H2 
=  5Fe2(SO4)3  +   2MnSO4  +   K2SO4  +  8H2O. 

Let  us  suppose  that  we  have  weighed  out  n  gram  of 
wire  and  dissolved  it  in  250  c.c.  of  liquid  :  50  c.c.  of  the 
solution  would  be  equivalent  to  0-2200  gram  of  iron.  The 
iron  we  have  taken,  however,  is  not  chemically  pure  :  we 
may  assume  without  sensible  error  that  its  impurities  amount 
to  0-4  per  cent.  ;  accordingly  the  amount  of  iron  in  the  50 
c.c.  is 

i    :  0-996  ::  0-2200   :   x.  $  =  0-2192. 

Let  us  further  suppose  that  we  have  required  20  c.c.  of 
permanganate  solution,  as  the  mean  of  the  experiments,  be- 
fore we  obtained  a  permanent  coloration  with  the  50  c.c.  of 
iron  solution :  then  20  c.c.  permanganate  oxidise  0*2192  gram 
iron  from  protoxide  to  peroxide,  or  100  c.c.  permanganate 
are  equivalent  to  i  "096  gram  iron. 

The  strength  of  the  permanganate  solution  may  also 
be  determined  by  means  of  pure  ferrous  sulphate,  precipitated 
from  its  aqueous  solution  by  means  of  alcohol.  The  ferrous 
sulphate  so  prepared  keeps  unchanged  for  years.  Or,  in- 


Use  of  Potassium  Permanganate.  151 

stead  of  this  salt,  the  double  sulphate  of  iron  and  ammonium 
FeSO4(NH4)2SO4  +  6H2O  may  be  employed.  It  contains 
exactly  one-seventh  of  its  weight  of  iron :  07  gram  of  salt  is 
equivalent  to  0*1  gram  of  iron. 

The  strength  of  the  solution  may  also  be  estimated  by 
means  of  oxalic  acid.  If  potassium  permanganate  solution 
is  dropped  into  a  solution  of  oxalic  acid  acidulated  with  sul- 
phuric acid,  the  oxalic  acid  is  completely  decomposed  into 
carbon  dioxide  and  water : 

5C2H204  +   2KMn04  +   3H2SO4 
K2SO4  +   2MnSO4  +   ioCO2  +  8H2O. 

The  oxalic  acid  solution  requires  to  be  gently  heated  (to 
about  60°)  before  the  reaction  commences :  at  this  tempera- 
ture the  permanganate  is  rapidly  decomposed  so  long  as  any 
oxalic  acid  remains  in  the  solution.  From  the  foregoing 
equations  it  is  evident  that  112  parts  of  iron  (in  the  state 
of  protoxide)  and  126  parts  of  crystallised  oxalic  acid 
(C2H2O4  +  2H2O)  require  exactly  the  same  amount  of 
oxygen,  viz.  16  parts,  for  complete  oxidation.  The  amount 
of  available  oxygen  in  i  c.c.  of  the  permanganate  solution 
may  therefore  be  readily  calculated. 

Of  the  several  substances  which  may  be  used  for  titrating 
the  permanganate  solution,  metallic  iron  is  on  the  whole  to  be 
preferred.  All  the  methods  are  equally  accurate  with  careful 
manipulation,  but  fewer  sources  of  error  attend  the  use  of 
metallic  iron.  Crystallised  oxalic  acid  is  not  readily  obtained 
quite  pure,  it  is  liable  to  part  with  a  portion  of  its  water  of 
crystallisation,  and  its  solution,  especially  under  the  influence 
of  light,  is  apt  to  decompose.  Ammonium  oxalate  is  prefer- 
able to  oxalic  acid  :  it  may  be  readily  purified  by  recrystal- 
lisation,  and  keeps  perfectly  well  :  its  composition  is  C2O4 
(NH4)2  +  H2O.  142-08  parts  of  the  crystallised  salt  are 
equivalent  to  1 1 2  parts  of  iron. 

Whichever  method  be  adopted  it  is  absolutely  necessary 
that  the  solution  to  be  titrated  should  contain  free  sulphuric 


152  Quantitative  Chemical  A  nalysis. 

acid.  If  there  is  a  deficiency  of  free  acid,  the  solution 
becomes  brown,  and  eventually  a  precipitate  is  formed.  It 
is  not  a  matter  of  indifference  which  acid  is  employed  for 
acidulation  :  nitric  acid  cannot  well  be  used  under  any 
circumstances,  and  hydrochloric  acid  is  liable  to  be  decom- 
posed, and  chlorine  eliminated,  in  accordance  with  the 
equation  : 

I4HC1    4-   Mn2O7    =    2MnCl2  +   5C12  +   7H2O. 

Whenever,  therefore,  the  use  of  sulphuric  acid  is  inadmis- 
sible, it  is  better  to  employ  potassium  bichromate  as  an 
oxidising  agent  (see  Analysis  of  Iron  Ores). 

XIV.    VOLUMETRIC  ESTIMATION  OF  CALCIUM  BY  MEANS 
OF  POTASSIUM  PERMANGANATE. 

Oxalic  acid  or  ammonium  oxalate  solution  added  to  cal- 
cium chloride  or  any  soluble  salt  of  lime,  in  presence  ol 
ammonium  chloride  and  free  ammonia,  gives  rise  to  a 
precipitate  of  calcium  oxalate.  Calcium  oxalate  digested 
with  dilute  sulphuric  acid  is  decomposed,  calcium  sulphate 
is  formed,  and  oxalic  acid  passes  into  solution.  We  already 
know  that  it  is  possible  to  determine  the  strength  of  a  per- 
manganate solution  by  means  of  an  oxalic  acid  solution  of 
known  strength  :  conversely  we  can  determine  an  unknown 
quantity  of  oxalic  acid  by  means  of  a  solution  of  perman- 
ganate, the  strength  of  which  is  accurately  known  to  us. 
Upon  these  considerations  is  based  a  method  of  determining 
lime  volumetrically. 

Weigh  out  about  2  grams  of  marble  into  a  250  c.c.  flask, 
dissolve  it  in  dilute  hydrochloric  acid,  heat  to  boiling,  add 
dilute  ammonia  in  slight  excess,  and  solution  of  ammonium 
oxalate  so  long  as  a  precipitate  is  formed.  Allow  the  pre- 
cipitate to  settle,  pour  the  supernatant  fluid  through  a  small 
filter,  and  thoroughly  wash  the  calcium  oxalate  by  decanta- 
tion  with  hot  water.  Pour  the  turbid  liquid  through  the 
filter,  but  do  not  bring  more  of  the  precipitate  on  the  filter 


Use  of  Potassium  Permanganate.  153 

than  you  can  help.  Wash  the  filter  thoroughly,  place  the 
funnel  in  the  neck  of  the  J-litre  flask,  and  pour  into  it  a 
quantity  of  dilute  sulphuric  acid,  previously  heated.  Again 
wash  the  filter  until  the  filtrate— which  should  of  course  drop 
into  the  flask — is  no  longer  acid.  Add  a  further  quantity  of 
dilute  sulphuric  acid  to  the  flask,  dilute  with  water,  and  heat 
gently.  Allow  the  liquid  to  cool,  fill  up  the  flask  to  the  con- 
taining-mark,  agitate,  and  quickly  transfer  100  c.c.  of  the 
turbid  liquid  to  a  beaker,  heat  to  about  60°,  and  add  the 
permanganate  solution  until  the  pink  colour  is  permanent. 
Again  shake  the  liquid  in  the  flask,  and  repeat  the  deter- 
mination on  a  second  quantity  of  100  c.c.  The  results 
should  agree,  i  eq.  of  oxalic  acid  is  equal  to  i  eq.  of  calcium. 
The  details  of  the  calculation  need  no  explanation. 

This  method  is  especially  convenient  when  a  number  of 
estimations  of  lime  have  to  be  made  in  succession.  It  may 
be  accelerated  by  adding  an  excess  of  ammonium  oxalate 
of  known  strength  to  the  solution  of  the  lime  salt, 
diluting  to  a  definite  bulk,  allowing  the  precipitate  to 
settle,  withdrawing  an  aliquot  portion  of  the  clear  liquid, 
acidulating  with  sulphuric  acid,  heating  to  60°,  and  adding 
the  permanganate  until  the  coloration  shows  that  the 
reaction  is  finished.  We  of  course  know  the  amount  of 
ammonium  oxalate  we  have  used  originally;  the  titration 
tells  us  the  excess  remaining  in  solution ;  the  difference 
expresses  that  combined  in  the  insoluble  lime  salt.  Since 
i  eq.  oxalic  acid  is  equal  to  i  eq.  of  calcium,  we  can  readily 
calculate  the  quantity  of  the  alkaline  earth  from  the  amount 
of  permanganate  solution  used. 

XV.     VOLUMETRIC  ESTIMATION  OF  LEAD  BY  PERMAN- 
GANATE SOLUTION. 

In  certain  cases  lead  may  be  estimated  by  the  foregoing 
methods,  but  on  account  of  the  slight  solubility  of  the  lead 
oxalate  in  water  containing  ammonia,  the  results  are  not 
quite  so  accurate  as  in  the  case  of  lime. 


1 54  Quantitative  Chemical  A  nalysis. 

XVI.  VALUATION  OF  MANGANESE  ORES  BY  MEANS  OF 

POTASSIUM  PERMANGANATE  SOLUTION. 

(See  Part  IV.) 

XVII.  ESTIMATION  OF  POTASSIUM  FERROCYANIDE  BY 

PERMANGANATE  SOLUTION. 

Potassium  permanganate  solution  added  to  a  solution  of 
potassium  ferrocyanide,  acidulated  with  sulphuric  acid, 
converts  that  salt  into  potassium  ferricyanide.  If  the  solu- 
tion is  sufficiently  dilute  there  is  no  difficulty  in  perceiving 
the  termination  of  the  reaction.  It  is  advisable  to  titrate  the 
permanganate  solution  (which  should  contain  about  i  gram 
KMnO4  per  1000  c.c.)  with  a  solution  of  potassium  ferro- 
cyanide of  known  strength;  5  grams  of  the  recrystallised 
salt  (K4FeCy6  +  3H2O)  dissolved  in  500  c.c.  of  water  forms 
a  convenient  solution.  A  measured  quantity  of  the  solution, 
say  25  c.c.,  is  placed  in  a  porcelain  basin  and  diluted  with 
about  ten  times  its  bulk  of  water,  together  with  a  quantity 
of  pure  sulphuric,  acid,  and  the  potassium  permanganate  is 
added  from  a  burette,  with  constant  stirring,  until  the  pure 
yellow  colour  of  the  solution  changes  to  a  reddish-yellow 
tint.  To  determine  the  amount  of  pure  ferrocyanide  in  a 
sample  of  the  article,  weigh  off  about  3  grams,  dissolve  in 
water,  and  make  up  the  solution  to  250  c.c.  25  c.c.  of  the 
liquid  are  transferred  to  a  porcelain  basin,  diluted  with 
water,  and  treated  in  the  manner  directed. 

XVIII.  ESTIMATION  OF  POTASSIUM  FERRICYANIDE  BY 

PERMANGANATE  SOLUTION. 

This  substance  may  be  analysed  by  reducing  it  to  the 
state  of  ferrocyanide,  and  determining  the  amount  of  the 
reduced  salt  in  the  manner  already  described.  The  weighed 
quantity  of  the  ferricyanide  is  rendered  strongly  alkaline  with 
caustic  potash  solution,  heated  to  boiling,  and  mixed  with  a 
concentrated  solution  of  ferrous  sulphate,  added  little  by 


Use  of  Iodine,  &c.  155 

little  until  the  colour  of  the  precipitate  is  black,  owing  to 
the  formation  of  triferric  tetroxide.  The  alkaline  solution 
is  diluted,  filtered,  and  the  filtrate  made  up  to  300  c.c. ;  50 
or  100  c.c.  are  then  transferred  to  a  porcelain  basin, 
strongly  acidified  with  sulphuric  acid,  and  titrated  with 
potassium  permanganate  solution. 


B.  ANALYSES  BY  MEANS  OF  IODINE  AND  SODIUM 
THIOSULPHATE  (HYPOSULPHITE)  SOLUTIONS. 

When  iodine  is  brought  into  contact  with  sodium  thio- 
sulphate  (hyposulphite)  the  following  reaction  occurs — 

2Na2S2O3  +  I2  =  Na2S4O6  +  2NaI 
iodine  and  sodium  thiosulphate  forming  sodium  tetrathionate 
and  sodium  iodide.  If  now  we  mix  with  the  thiosulphate 
solution  a  small  quantity  of  starch,  and  add  the  iodine  so- 
lution drop  by  drop,  the  sensitive  blue  colour  of  the  iodide  of 
starch  will  continue  to  be  destroyed  as  fast  as  it  is  produced, 
so  long  as  the  foregoing  reaction  occurs.  Immediately  that 
the  whole  of  the  thiosulphate  has  been  converted  into 
tetrathionate,  the  least  excess  of  iodine  will  act  upon  the 
starch,  and  the  blue  colour  will  be  permanent.  If,  therefore, 
we  have  a  solution  of  thiosulphate  of  known  strength  we  can 
readily  estimate  the  amount  of  iodine  in  a  solution  containing 
an  unknown  quantity  of  that  element 

A  solution  of  iodine,  in  presence  of  substances  which 
readily  take  up  oxygen,  decomposes  water,  forming  hydriodic 
acid  with  its  hydrogen  and  giving  up  the  oxygen  to  the 
oxidisable  substance.  Thus  in  the  case  of  arsenious  acid — 

As2O3  +   2l2   +   2H2O     =     4HI   +  As2O5. 
In  the  case  of  sulphur  dioxide — 

SO2  +  I2  +  2H2O    =   SO4H2   +    2HI. 

These  reactions  afford  the  basis  of  an  exact  and  generally 
applicable  volumetric  process. 


1 56  Quantitative  Chemical  A  nalysis. 

The  method  requires — 

(1)  A  solution  of  iodine  of  known  strength. 

(2)  A  solution  of  sodium  thiosulphate  of  known  strength. 

(3)  A  solution  of  starch. 

i.  Preparation  of  the  Iodine  Solution. — This  may  most 
conveniently  be  of  deci-normal  strength  :  i  c.c.  should  con- 
tain therefore  0-012685  gram  iodine.  About  13  grams  of 
pure  iodine  are  weighed  out  into  a  litre  flask,  together  with 
about  1 8  grams  of  pure  potassium  iodide,  the  whole  is  dis- 
solved in  about  300  c.c.  of  perfectly  cold  water,  and  diluted 
so  as  to  be  of  deci-normal  strength.  Thus  supposing  that  we 
had  weighed  out  exactly  13  grams  of  iodine,  we  should 
require  to  dilute  the  liquid  to  1024  c.c.,  since 

i2'685   :    1000    ::    13   \  x  x  =  1024. 

The  pure  iodine  may  be  obtained  by  intimately  mixing  re- 
sublimed  iodine  with  about  one-fourth  of  its  weight  of 
powdered  potassium  iodide,  heating  the  mixture  in  a  large 
porcelain  crucible  placed  on  an  iron  plate,  and  surmounted 
by  a  precisely  similar  crucible.  The  resublimed  crystals  are 
loosened  from  the  sides  of  the  crucible  and  placed  over 
strong  oil  of  vitriol  within  a  bell-jar  for  a  few  hours,  in  order 
to  deprive  them  of  the  last  traces  of  hygroscopic  moisture. 
The  dried  iodine  is  then  quickly  transferred  to  a  clean  dry 
test-tube,  into  which  a  second  test-tube  is  fitted,  in  the 
manner  seen  in  fig.  49.  The  stoppered 

FIG.  49.        FIG.  50.         ,  ,    .       -  iv 

tube  represented  in  fig.  50  may  also  be 
employed.  The  tubes  containing  the 
iodine  are  accurately  weighed,  a  portion  of 
the  substance  is  transferred  to  the  litre- 
flask  containing  the  potassium  iodide,  dis- 
solved in  water,  and  the  tubes  are  again 
weighed.  The  loss  of  weight  gives  the 
amount  of  iodine  taken  :  care  should  be 
taken  that  the  amount  is  not  less  than 
12-685  grams.  Any  powder  adhering  to  the  sides  of  the 


Use  of  Iodine,  &c.  157 

flask  is  washed  down  into  the  liquid ;  when  the  whole  of  the 
iodine  is  dissolved  the  solution  is  diluted  to  the  proper 
degree.  The  liquid  must  not  be  heated  ;  if  the  weighing 
out  and  the  dilution  have  been  carefully  conducted  the 
solution  will  be  strictly  deci-normal.  The  solution  is  most 
conveniently  preserved  in  small  stoppered  bottles  of  about 
200  c.c.  capacity,  which  should  be  filled  to  the  neck  and 
kept  in  a  cool,  dark  place. 

2.  Preparation  of  Deci-normal  Solution  of  Sodium  Thiosul- 
phate.— i  litre  contains  24*8  grams  of  the  salt.     About  25 
grams  of  the  recrystallised  salt,  previously  powdered  and 
dried  between  filter-paper,  are  accurately  weighed  out  into 
the  litre  flask,  dissolved  in  water,  and  the  solution  diluted  so 
that  i  c.c.  =  0-0248  gram  of  thiosulphate.     The  solution 
should  also  be  kept  in  the  dark  :  when  exposed  to  light,  it 
slowly    decomposes,    with    the    precipitation    of   sulphur. 
Accordingly  a  fresh  solution  of  the  salt  should  be  prepared 
from  time  to  time. 

3.  Preparation  of  Starch  Solution. — i  gram  of  pure  wheaten 
starch  is  mixed  with  a  small  quantity  of  water  and  rubbed 
to  a  thin  cream  in  a  mortar.     The  paste  is  poured  into  150 
c.c.  of  boiling  water,  the  liquid  is  allowed  to  stand,  and  the 
clear  solution  decanted  from  the  sediment.     The  solution 
should  be  prepared  before  the  beginning  of  each  series  of 
experiments,  since  it  decomposes  after  a  time.     By  adding 
about  10  c.c.  of  glycerine  to  it,  or  saturating  it  with  common 
salt,  the  solution  keeps  better :  it  is  so  readily  prepared, 
however,  that  it  is  preferable  to  make  a  fresh  solution  when 
wanted. 

If  any  doubt  should  exist  as  to  the  exact  strength  of  the 
thiosulphate  solution,  it  may  be  readily  standardised  by  the 
aid  of  a  deci-normal  solution  of  potassium  bichromate. 
Potassium  bichromate  solution  added  to  potassium  iodide, 
in  presence  of  free  hydrochloric  acid  liberates  iodine  : 
K2Cr2O7  +  6KI  +  I4HC1  =  3X2  +  8KC1  +  Cr2Cl6+7H2O. 


158  Quantitative  Chemical  Analysis. 

294-3  parts  of  potassium  bichromate  liberate  761-1  parts  of 
iodine  :  accordingly  i  c.c.  of  deci-normal  solution  of 
bichromate  liberates  0-012685  gram  of  iodine. 

25  c.c.  of  deci-normal  bichromate  solution  made  by  dis- 
solving 4-907  grams  of  potassium  bichromate  in  a  litre  of 
water  are  placed  in  a  flask  and  mixed  with  about  10  c.c.  of 
solution  of  pure  potassium  iodide  (i  of  salt  to  10  of  water), 
together  with  about  5  c.c.  of  pure  hydrochloric  acid,  and  200 
c.c.  of  water.  The  standard  thiosulphate  solution  is  then 
added  drop  by  drop  until  the  iodine  has  nearly  disappeared : 
a  few  drops  of  starch  solution  are  added,  and  the  addition  of 
the  thiosulphate  continued  until  the  last  trace  of  the  blue 
colour  of  the  iodide  of  starch  vanishes.  Since  the  25  c.c.  of 
bichromate  solution  liberate  25  x  0-012685  =  0-3171  gram 
iodine,  we  can  readily  calculate  the  amount  of  iodine 
equivalent  to  i  c.c.  of  the  sodium  thiosulphate  solution. 

XIX.  VALUATION  OF  BLEACHING-POWDER  BY  IODINE 
AND  SODIUM  THIOSULPHATE  SOLUTIONS. 

(See  Part  IV.) 

XX.  ESTIMATION  OF  THE  AMOUNT  OF  CHLORINE  IN 

AQUEOUS  SOLUTIONS  OF  THE  GAS. 

Prepare  a  dilute  solution  of  chlorine  in  water,  measure  off 
a  definite  quantity  of  the  liquid,  and  transfer  it  to  a  solu- 
tion of  potassium  iodide.  Determine  the  amount  of  the 
liberated  iodine  by  solution  of  sodium  thiosulphate,  adding 
the  latter  liquid  until  the  iodine  has  nearly  disappeared  ;  add 
2  or  3  c.c.  of  starch  solution,  and  continue  the  addition  of 
the  thiosulphate  until  the  blue  colour  just  vanishes. 

Add  a  second  portion  of  the  chlorine  water  to  the  solu- 
tion of  potassium  iodide,  and  a  measured  quantity  (in 
excess)  of  sodium  thiosulphate,  and  determine  the  amount 
of  the  residual  thiosulphate  by  means  of  starch  and  stan- 
dard iodine  solution,  adding  the  latter  until  the  blue  colour 
of  the  solution  is  persistent.  The  results  of  the  two  expe- 
riments should  agree. 


Use  of  Iodine,  &c.  159 

XXI.    ESTIMATION  OF  THE  AMOUNT  OF  SULPHUR  DIOXIDE 
IN  AQUEOUS  SOLUTIONS  OF  THE  GAS. 

Prepare  a  very  dilute  solution  of  sulphur  dioxide  (by 
adding  10  c.c.  of  a  saturated  solution  of  the  gas  to  a  litre  of 
water),  transfer  a  definite  quantity  of  the  liquid  to  a  known 
amount  (in  excess)  of  standard  iodine  solution,  and  deter- 
mine the  amount  of  residual  iodine  by  starch  and  sodium 
thiosulphate  solution. 

To  a  second  measured  portion  of  the  solution  of  the  gas, 
add  2  or  3  c.c.  of  starch  solution,  and  the  standard  solution 
of  iodine,  until  the  blue  colour  of  the  iodide  of  starch  is 
persistent.  The  reaction  between  sulphur  dioxide  and 
iodine  may  be  thus  represented  : 

SQ2  +  I2  +  2H20     =     S04H2  +  2HI. 

If,  however,  the  solution  is  concentrated,  the  sulphuric 
acid  reacts  upon  the  hydriodic  acid,  and  sulphur  dioxide  and 
free  iodine  are  again  formed — 

SO4H2  +   2HI     =     SO2  +   I2  +   2H2O. 

So  long  as  the  solution  contains  only  about  0*05  per  cent, 
of  sulphur  dioxide,  the  first  reaction  alone  takes  place. 

XXII.     ESTIMATION  OF  SULPHURETTED  HYDROGEN  IN 
AQUEOUS  SOLUTIONS  OF  THE  GAS. 

When  sulphuretted  hydrogen  is  brought  into  contact  with 
free  iodine,  the  following  reaction  ensues  : — 

H2S  +  I2     =     2HI  +  S. 

This  reaction,  however,  is  liable  to  be  modified  by  the 
concentration  of  the  solution  ;  experiment  has  shown  that 
it  can  only  be  depended  upon  when  the  solution  contains 
not  more  than  0^04  per  cent,  of  the  gas. 

Prepare  a  dilute  solution  of  sulphuretted  hydrogen, 
measure  off  a  definite  quantity,  add  starch  liquor  and  solu- 
tion of  iodine  until  the  colour  is  persistent. 


160  Qitantitative  Chemical  Analysis. 

Place  in  a  flask  about  the  same  quantity  of  iodine  solu- 
tion which  you  have  consumed  in  the  foregoing  experiment, 
and  then  add  a  measured  quantity  (in  excess)  of  the  solution 
of  sulphuretted  hydrogen.  Determine  the  excess  of  sul- 
phuretted hydrogen  by  starch  and  standard  iodine  solutions. 

The  amount  of  sulphuretted  hydrogen  in  mineral  waters 
may  be  readily  determined  by  this  method.  For  such  esti- 
mations it  will  be  more  convenient  to  dilute  the  iodine  solu- 
tion to  ten  times  its  bulk  ;  100  c.c.  are  transferred  to  the 
litre  flask,  and  the  flask  is  filled  up  to  the  containing-mark. 

Measure  off  250  c.c.  of  the  mineral  water,  transfer  to  a 
beaker,  add  i  or  2  c.c.  of  starch  liquor  and  centi-normal 
solution  of  iodine,  until  the  blue  colour  is  persistent.  Let 
us  suppose  we  have  used  20  c.c.  of  centi-normal  solution  of 
iodine ;  this  amount  of  the  iodine  solution  is  placed  in 
a  beaker,  and  mixed  with  250  c.c.  of  the  mineral  water ; 
i  or  2  c.c.  of  starch  liquor  are  added,  and  then,  cautiously, 
centi-normal  solution  of  iodine,  drop  by  drop,  until  the  blue 
colour  of  the  iodide  of  starch  remains.  It  will  be  found  in 
general  that  i  or  2  c.c.  more  of  iodine  solution  are  required 
in  the  second  experiment  than  in  the  first.  Now  pour 
250  c.c.  of  distilled  water  into  the  beaker,  add  the  same 
bulk  of  starch  liquor  used  in  the  last  experiment,  and,  drop 
by  drop,  the  solution  of  iodine,  until  the  blue  colour  is  pro- 
perly defined.  This  experiment  gives  the  amount  of  the 
iodine  solution  required  to  produce  the  final  reaction ;  subtract 
this  quantity  from  the  amount  of  iodine  solution  required  in 
the  second  experiment.  The  remainder  shows  the  amount 
of  iodine  solution  equivalent  to  the  sulphuretted  hydrogen 
present  ;  this  amount  may  readily,  be  calculated  from  the 
above  equation. 

XXIII.     ESTIMATION  OF  HYDROCYANIC  ACID. 

When  potassium  cyanide  is  mixed  with  solution  of  iodine, 
iodide  of  potassium  and  iodide  of  cyanogen  are  formed  : — 

KCy  +   I2     =     KI   +   Cyl. 


Use  of  Iodine,  &c.  1 6 1 

Two  eq.  or  2537  parts  of  iodine  correspond  to  i  eq.  or 
65*17  of  potassium  cyanide.  This  principle  affords  the 
basis  of  an  exact  method  for  determining  the  value  of 
potassium  cyanide,  or  solutions  of  prussic  acid. 

In  the  case  of  potassium  cyanide,  weigh  out  about 
2  grams  of  the  salt  into  a  J-litre  flask,  dissolve  in  water, 
dilute  to  the  mark,  shake,  and  transfer  50  c.c.  to  a  beaker 
containing  about  200  c.c.  of  water,  add  100  c.c.  of  a  satu- 
rated solution  of  carbonic  acid  gas  in  water  (to  convert 
the  alkaline  carbonates,  always  present  in  the  commercial 
article,  to  acid  carbonates),  and  add  solution  of  iodine,  until 
the  liquid  possesses  a  slight  and  permanent  yellow  tinge. 
The  cyanide  must  be  free  from  alkaline  sulphide. 

In  the  case  of  free  hydrocyanic  acid,  add  a  very  slight 
excess  of  caustic  soda,  and  then  solution  of  carbonic  acid, 
and  proceed  in  the  manner  above  described.  The  specific 
gravity  of  the  ordinary  preparations  of  hydrocyanic  acid  is  so 
little  removed  from  that  of  water,  that  it  is  more  prudent  to 
determine  the  amount  taken  for  analysis  by  weighing,  rather 
than  by  measuring  the  solution  in  a  pipette,  by  aspiration  in 
the  ordinary  manner. 

XXIV.  ESTIMATION  OF  ANTIMONY  IN  TARTAR  EMETIC, 
AND  OF  ARSENIC  AND  ARSENIOUS  ACIDS  IN  COM- 
MERCIAL ARSENIATES. 

These  methods  depend  upon  the  conversion  of  antimony 
or  arsenic  trioxide,  in  an  alkaline  solution,  into  pentoxide,  by 
solution  of  iodine : 

Sb2O3  +  2l2  +  2H2O    ==    Sb2O5  +  4HI 
As2O3  +  2l2  +  2H2O    =    As2O5  +  4HI 

Weigh  out  about  2  grams  of  the  tartar  emetic,  dissolve  in 
water,  and  dilute  to  250  c.c.  Transfer  20  c.c.  of  the  solu- 
tion to  a  beaker,  add  the  same  amount  of  a  saturated 
solution  of  sodium  bicarbonate,  and  2  c.c.  of  starch  liquor. 
Now  add  the  iodine  solution  until  the  blue  colour  is 

M 


1 62  Quantitative  Chemical  Analysis. 

persistent  for  about  5  minutes.  The  fluid  acquires  a  reddish 
tint  just  before  the  reaction  is  completed  :  the  blue  coloura- 
tion, even  after  the  reaction  is  finished,  fades  after  a  time,  say 
in  15  minutes. 

In  the  case  of  commercial  arseniates  weigh  out  about  3 
grams  of  the  substance  into  a  -J-litre  flask,  and  dissolve  in 
about  150  c.c.  of  warm  water,  add  a  small  quantity  of 
sodium  acetate  and  acetic  acid,  and  boil  the  solution  for  15 
minutes  to  decompose  any  nitrites  :  when  cold  make  up  to 
500  c.c. 

To  determine  the  amount  of  the  arsenite  withdraw  50  c.c. 
of  the  solution,  add  25  cb.c.  of  a  saturated  solution  of  pure 
NaHCO3,  starch  paste,  and  decinormal  iodine  solution 
(i  c.c.  =  "00495  gr&m  As2O3). 

Now  pass  sulphur  dioxide  gas  into  the  solution  in  the 
flask  to  reduce  the  pentoxide 

As2O5  +  2SO2  +  2H2O  =  As2O3  +  2H2SO4, 

boil  to  expel  the  excess  of  sulphur  dioxide,  and  when  cold 
make  up  to  500  c.c.  Withdraw  50  c.c.  of  the  solution,  add 
25  c.c.  of  the  sodium  bicarbonate  solution,  starch  paste  and 
iodine  solution  as  before  (i  c.c.  =0-00575  gram  As2O5). 

XXV.     DETERMINATION  OF  TIN  BY  IODINE  SOLUTION. 

The  metal  is  dissolved  in  hydrochloric  acid,  and  mixed 
with  a  solution  of  Rochelle  salt,  and  a  concentrated  solution 
of  sodium  bicarbonate  is  then  added  until  the  liquid  is  no 
longer  acid.  Add  about  i  c.c.  of  starch  solution  and  deci- 
Rorinal  solution  of  iodine,  until  the  blue  colour  is  persistent. 
2537  parts  of  iodine  are  equivalent  to  118  of  tin.  The 
solution  of  the  metal  is  best  effected  in  a  stream  of  carbon 
dioxide :  the  addition  of  a  few  scraps  of  platinum-foil  ac- 
celerates the  process. 

This  method  is  of  course  applicable  to  the  valuation  of 
c  Tin  crystals.' 


Use  of  Iodine,  &c.  1 63 

ANALYSES  BY  MEANS  OF  IODINE  AND  SODIUM 
THIOSULPHATE  SOLUTIONS,  WITH  PREVIOUS 
DISTILLATION  WITH  HYDROCHLORIC  ACID. 

A  number  of  substances  containing  oxygen,  when  heated 
with  hydrochloric  acid,  are  decomposed  in  such  manner  that 
free  chlorine  is  evolved  in  amount  bearing  some  simple  ratio 
to  the  quantity  of  oxygen  present.  Thus  when  manganese 
dioxide  is  heated  with  hydrochloric  acid,  the  following 
reaction  occurs : 

Mn02  +  4HC1  =  MnCl2  +  C12  +  2H2O 
70-92  parts  of  chlorine  are  therefore  equivalent  to  86-04  of 
manganese  dioxide.  If  the  chlorine  be  led  into  a  solution 
of  potassium  iodide,  iodine  will  be  liberated  in  exact  propor- 
tion to  the  chlorine  evolved  :  the  amount  of  iodine  liberated 
is  a  measure  therefore  of  the  amount  of  real  manganese 
dioxide  in  the  sample  analysed. 

Similarly  when  potassium  bichromate  is  heated  with 
hydrochloric  acid,  chlorine  is  evolved  : 

K2Cr207  +  i4HCl  =  Cr2Cl6  +  ;H2O  +  2KC1  +  3C12. 
212-76  parts  of  chlorine  are  equivalent  therefore  to  294-3 
parts  of  potassium  bichromate.  As  we  have  seen  from  the 
method  of  titration  given  on  p.  158,  No.  XX.  the  disengaged 
chlorine,. in  presence  of  excess  of  potassium  iodide,  liberates 
an  equivalent  quantity  of  iodine,  which  can  be  estimated  by 
means  of  starch  and  sodium  thiosulphate  solutions. 

XXVI.    ANALYSIS  OF  POTASSIUM  BICHROMATE. 

Into  the  little  bulb  a,  which  has  a  capacity  of  about  60  c.c. 
(fig.  50 A),  accurately  weigh  out  about  0*3  or  0-4  gram  of  pure 
fused  potassium  bichromate,  add  about  25  c.c.  of  pure  fuming 
hydrochloric  acid,  and  connect  the  bulb  with  the  bent  tube 
b  by  means  of  a  tightly-fitting  caoutchouc  tube,  which  has 
been  previously  boiled  in  caustic  soda  solution  to  remove 
any  adhering  sulphur.  The  bent  tube  is  connected  with  a  two- 
bulb  U-tube  by  means  of  a  caoutchouc  cork,  which  should 

M  2 


164 


Quantitative  Chemical  A  nalysis. 


also  have  been  previously  cleansed  by  caustic  soda  solution. 
In 'certain  cases  it  is  advisable  to  have  a  second  U-tube, 
which  is  connected  with  the  first  by  corks  and  a  bent  tube. 
Both  the  tubes  are  placed  in  a  beaker  and  are  surrounded 
by  cold  water:  they  each  contain  about  25  c.c.  of  strong 
solution  of  potassium  iodide.  Gently  heat  the  bulb  con- 
taining the  bichromate  and  acid ;  chlorine  is  readily  evolved 

FIG.  SOA. 


and  decomposes  the  potassium  iodide  in  the  U-tubes: 
after  two  or  three  minutes'  heating,  the  whole  of  the  chlorine 
will  have  been  eliminated.  Heat  the  liquid  to  boiling,  so  as 
to  drive  over  the  last  traces  of  the  gas ;  remove  the  lamp, 
and  allow  the  whole  to  stand  for  5  or  10  minutes  to  effect 
the  complete  absorption  of  the  chlorine ;  empty  the  contents 
of  the  U-tubes,  when  quite  cold,  into  a  beaker  (the  solution 
in  the  second  tube  need  not  be  added  unless  it  contains 
liberated  iodine),  dilute  with  water,  and  titrate  with .  starch 
and  thiosulphate  solution. 


Use  of  Iodine,  &c.  165 

XXVII.     ESTIMATION  OF  ARSENIOUS  ACID  BY  THE  AID 

OF   THE    FOREGOING   REACTION. 

If  we  mix  the  weighed  quantity  of  bichromate  with  finely- 
powdered  arsenious  acid  (not  in  excess)  only  a  portion  of 
the  chlorine  demanded  by  the  equation  is  evolved;  the 
deficit  has  served  to  bring  about  the  oxidation  of  the  arsenic 
trioxide  to  pentoxide : 

As2O3  +  4C1  +  2H2O  =  As2O5  -f  4HC1. 
Consequently  every  4  eq.  of  missing  chlorine,  i.e.  less  than 
that  which  would  be  obtained  by  distilling  the  weighed 
amount  of  bichromate  alone  with  hydrochloric  acid,  represent 
i  eq.  of  arsenic  trioxide.  The  details  of  the  method  are 
identical  with  those  of  the  foregoing  example. 

XXVIII.    ANALYSIS  OF  CHLORATES,  BROMATES,  AND 
IODATES. 

i  eq.  of  potassium  chlorate,  heated  with  a  large  excess  of 
hydrochloric  acid,  evolves  an  amount  of  chlorine  partly 
free  and  partly  in  combination  with  oxygen  sufficient  to 
liberate  6  eq.  of  iodine.  761*1  parts  of  iodine  are  therefore 
equivalent  to  122-56  of  potassium  chlorate.  The  weighed 
quantity  of  the  chlorate  (about  0*2  gram  is  sufficient)  is 
placed  in  the  distillation  flask  and  heated  with  excess  of 
hydrochloric  acid.  The  remainder  of  the  operation  is  con- 
ducted in  the  manner  described.  Bromates  and  iodates  are 
best  analysed  by  the  method  of  digestion  instead  of  by  that 
of  distillation.  A  strong  bottle  of  about  120-150  c.c.  is 
fitted  with  an  accurately-ground  stopper:  the  weighed 
amount  of  the  bromate  is  placed  in  the  bottle,  the  requisite 
amount  of  a  saturated  solution  of  potassium  iodide  and 
hydrochloric  acid  is  added,  and  the  stopper  is  firmly  fastened 
down  by  binding  wire.  The  bottle  is  then  placed  on  the 
water-bath ;  when  the  decomposition  is  complete  it  is  allowed 
to  cool,  and  its  contents  are  diluted  with  water ;  the  solution 
is  emptied  into  a  beaker,  and  the  titration  proceeded  with 
in  the  usual  manner. 


1 66  Quantitative  Chemical  A  nalysis. 

In  the  case  of  iodates  and  bromates,  only  4  eq.  of  iodine 
are  liberated  for  each  eq.  of  the  acid. 

The  method  of  digestion  may  be  frequently  substituted 
for  that  of  distillation.  It  is  of  course  absolutely  necessary 
that  the  stopper  of  the  bottle  fits  perfectly:  to  test  it,  it 
should  be  tied  down  in  the  empty  bottle,  which  is  then  to 
be  immersed  in  hot  water ;  if  any  air-bubbles  make  their 
escape  between  the  stopper  and  the  neck,  the  bottle  is 
useless  for  this  purpose.  The  stopper,  if  nearly  tight,  may  be 
re-ground  with  a  little  fine  emery  and  water.  In  every  case 
it  must  be  carefully  tested  before  use. 

XXIX.    ESTIMATION  OF  IRON  BY  MEANS  OF  IODINE  AND 
THIOSULPHATE  SOLUTIONS. 

When  ferric  chloride  is  added  to  a  warm  solution  of 
potassium  iodide  the  following  reaction  ensues  : 

Fe2Cl6  +  2KI     =     2FeCl2  +  2KC1  +  I2. 
126*85  parts  of  iodine  correspond  to  56  parts  of  iron. 

Dissolve  5*02  grams  of  clean  piano- wire  (corresponding 
to  5  grams  of  pure  iron)  in  dilute  hydrochloric  acid,  in  a 
^-litre  flask;  add  a  few  crystals  of  potassium  chlorate,  to 
convert  the  iron  into  ferric  chloride,  boil  the  solution  for 
some  time,  to  expel  the  excess  of  chlorine,  and  when  cold 
dilute  the  solution  to  the  containing-mark.  10  c.c.  of  the 
solution  correspond  to  o'i  gram  of  iron.  Transfer  20  c.c. 
of  the  iron  solution  to  the  stoppered  bottle,  cautiously  add 
dilute  caustic  soda  solution  until  a  slight  precipitate  of  oxide 
of  iron  remains,  and  then  i  c.c.  of  hydrochloric  acid 
(sp.  gr.  i'i).  Now  add  about  4  grams  of  potassium  iodide 
to  the  clear  dark-yellow  solution,  insert  the  stopper,  and 
fasten  it  down  by  means  of  binding  wire.  Heat  the  bottle 
over  the  water-bath,  for  ten  or  fifteen  minutes,  to  about  60°, 
allow  it  to  cool  completely,  open  it,  and  add  sodium  thio- 
sulphate  from  a  burette,  until  the  solution  is  nearly  de- 
colourised, add  i  c.c.  of  starch  liquor,  and  continue  the 
addition  of  the  thiosulphate  until  the  blue  colouration  just 
disappears. 


Use  of  Iodine,  &c.  1 67 

This  method  is  particularly  useful  for  the  determination  of 
small  quantities  of  iron.  The  solution  must,  of  course,  be 
fully  oxidised,  and  contain  no  other  substance  which  can 
eliminate  iodine.  If  strongly  acid,  it  must  be  partially 
neutralised  by  soda  solution,  in  the  manner  described. 

XXX.     ESTIMATION  OF  NITRIC  ACID  BY  SOLUTIONS  OF 
IRON,  IODINE,  AND  SODIUM  THIOSULPHATE. 

Free  nitric  acid,  added  to  a  solution  of  ferrous  chloride, 
converts  the  iron  into  ferric  chloride.  Ferric  chloride,  as  we 
have  seen  in  the  foregoing  process,  may  be  estimated  by 
means  of  iodide  of  potassium  and  sodium  thiosulphate. 

Weigh  out  about  07  gram  of  iron- wire,  and  dissolve  it  in 
a  small  quantity  of  hydrochloric  acid,  in  a  flask  in  a 
current  of  carbon  dioxide,  in  order  to  prevent  the  least 
chance  of  oxidation.  Weigh  out  about  2  grams  of  nitre 
into  a  ^-litre  flask,  dissolve  in  boiling  water,  and  dilute 
with  boiled  water  to  the  containing-mark.  Transfer  25  c.c. 
of  the  solution,  corresponding  to  0-2  gram  of  nitre,  to  the 
iron  solution,  and  quickly  re-insert  the  cork ;  gently  heat  the 
liquid,  at  length  to  boiling,  to  expel  the  nitric  oxide.  Main- 
tain a  rapid .  current  of  carbon  dioxide  throughout  the 
operation.  As  soon  as  the  solution  is  of  a  pure  yellow 
colour,  allow  it  to  cool  in  a  current  of  carbon  dioxide,  add 
a  sufficiency  of  potassium  iodide,  allow  the  solution  to  stand 
for  a  short  time,  and  determine  the  amount  of  liberated 
iodine  by  starch  and  thiosulphate  solution,  exactly  as  de- 
scribed in  the  preceding  method.  The  quantity  of  thiosul- 
phate used,  multiplied  by  0*0021,  gives  the  amount  of  nitric 
acid  present. 

XXX.  VALUATION  OF  MANGANESE  ORES  BY  DISTILLATION 
WITH  HYDROCHLORIC  ACID,  AND  TITRATION  WITH  IODINE 
AND  THIOSULPHATE  SOLUTIONS. 

(See  Part  IV.) 


i68 


Quantitative  Chemical  A  nalysis. 


PART    IV. 

GENERAL  ANALYSIS,  INVOLVING  GRAVIMETRIC 
AND    VOLUMETRIC  PROCESSES. 

I.     NITRE. 

THE  crude  nitre  of  commerce  invariably  contains  alkaline 
chlorides  and  sulphates,  together  with  more  or  less  insoluble 
matter  and  moisture.  Samples  of  nitre  are  occasionally  met 
with  containing  sodium  nitrate,  arising  either  from  imperfect 
decomposition  of  the  Chili  saltpetre,  from  which  the  nitre 
was  prepared,  or  from  wilful  adulteration.  Such  nitre  is 
highly  hygroscopic,  and  requires  to  be  purified  before  it  can 
be  used  for  the  manufacture  of  gunpowder. 

The  presence  of  sodium  nitrate,  or  excess  01  common 
salt  in  the  nitre,  may  be  readily  detected  by  means  of  the 
spectroscope.  The  quantity  of  admixed  sodium  nitrate 
may  be  approximately  determined  by  ascertaining  the 
amount  of  water  taken  up  from  a  perfectly  moist  atmo- 
sphere. Pure  nitre  placed  over  the  surface  of  water  for 
a  fortnight  remains  comparatively  dry ;  sodium  nitrate, 
under  the  same  circumstances,  absorbs  one-fourth  of  its 
weight  of  water.  The  following  table  shows  the  amount  of 
water  taken  up  by  100  grams  of  the  mixed  nitrates  : — 


Percentage  of  Sodium  Ni-"l 
trate  .  .  .  / 

0*5 

I 

3 

5 

10 

Amount  of  Water  (in  grams)  "1 
absorbed  in  14  days  .  J 

2'5 

4 

10 

12 

19 

Determination  of  Moisture. — About   20    grams    of    the 
sample  are  gently  heated  in  a  weighed  platinum  crucible, 


Nitre.  169 

until  the  salt  commences  to  fuse.     When  cold,  the  crucible 
is  again  weighed.     The  loss  gives  the  amount  of  moisture. 

Determination  of  Insoluble  Matter. — The  contents  of  the 
crucible  are  washed  out  into  a  porcelain  basin  with  hot 
water,  dissolved,  filtered,  and  the  insoluble  matter  dried  and 
weighed.  The  filtrate  is  received  into  a  ^-litre  flask,  allowed 
to  cool,  and  diluted  to  the  containing- mark. 

Determination  of  Chlorine. — 100  c.c.  of  the  liquid,  corre- 
sponding to  4  grams  of  nitre,  are  transferred  to  a  porcelain 
basin,  and  the  chlorine  estimated  by  means  of  standard 
silver  nitrate  and  potassium  chromate  solutions.  If  the 
amount  of  chlorine  is  very  small,  it  is  advisable  to  use  centi- 
normal  silver  solution. 

Determination  of  Sulphuric  Acid. — 250  c.c.  of  the  liquid 
are  transferred  to  a  beaker,  heated  to  boiling,  and  acidu- 
lated with  a  small  quantity  of  hydrochloric  acid.  Barium 
chloride  solution  is  added,  and  the  liquid  is  set  aside  for  a 
time,  to  allow  the  precipitate  to  subside  perfectly.  As 
barium  sulphate  is  slightly  soluble  in  solutions  of  the 
alkaline  nitrates,  the  precipitation  is  not  quite  complete. 
The  clear  supernatant  liquid  is  poured  through  the  filter, 
and  the  precipitate  is  washed  several  times  by  decantation 
with  boiling  water,  taking  care  to  allow  it  to  settle  as  com- 
pletely as  possible,  before  pouring  the  liquid  on  to  the 
filter.  The  barium  sulphate  still  retains  co-precipitated 
nitrate.  This  may  be  best  removed  by  a  solution  of  copper 
acetate.  Crystallised  copper  acetate  is  dissolved  in  a  small 
quantity  of  hot  water,  containing  acetic  acid  ;  one  drop  of 
sulphuric  acid  is  added,  and  then  one  drop  of  barium  chlo- 
ride solution;  the  mixture  is  boiled  and  filtered.  About 
5  or  10  c.c.  of  the  saturated  solution,  according  to  the  amount 
of  the  precipitate,  is  added  to  the  barium  sulphate,  together 
with  a  small  quantity  of  water  and  a  few  drops  of  acetic 


170  Quantitative  Chemical  Analysis. 

acid.  The  liquid  is  heated  to  boiling,  and  maintained  in 
ebullition  for  ten  or  fifteen  minutes.  The  amount  of  acetic 
acid  added  should  be  sufficient  to  prevent  the  precipitation 
of  any  basic  salt  of  copper.  Pour  the  liquid  through  the 
filter,  transfer  the  precipitate,  and  wash  it  thoroughly  with 
hot  water.  Dry,  ignite,  and  weigh  it.  This  procedure  is 
recommended  to  be  followed  in  all  precipitations  of  sul- 
phuric acid  by  barium  salts,  in  presence  of  considerable 
quantities  of  alkaline  nitrates  or  chlorides. 

Determination  of  Nitric  Add. — Fuse  a  few  grams  of  the 
nitre  at  the  lowest  possible  temperature,  and  pour  out  the 
liquid  mass  into  a  warm  porcelain  dish,  powder  it  quickly, 
and  transfer  it  to  a  tube.  Place  2  or  3  grams  of  powdered 
quartz  in  a  platinum  crucible,  heat  to  redness,  and  weigh 
accurately  after  cooling.  Add  about  0-5  gram  of  the  nitre, 
and  again  weigh.  Mix  the  nitre  and  silica  by  the  aid  of  a 
thin  glass  rod,  taking  care,  of  course,  that  nothing  adheres 
to  the  rod,  and  heat  the  crucible  gradually  to  a  low  red  heat, 
keeping  it  at  this  temperature  for  twenty  or  thirty  minutes, 
transfer  to  the  desiccator,  and  weigh  when  cold.  The  loss 
of  weight  gives  the  amount  of  nitric  acid.  Care  must  be 
taken  to  regulate  the  temperature  properly,  or  the  sulphates 
and  chlorides  present  (particularly  the  latter)  may  partially 
volatilise.  Potassium  bichromate  and  borax  may  be  substi- 
tuted for  the  powdered  quartz ;  the  latter  substance,  how- 
ever, is  preferable. 

The  nitric  acid  may  also  be  determined  by  the  method 
described  on  p.  96. 

II.     GUNPOWDER. 

Gunpowder  is  an  intimate  mixture  of  sulphur,  nitre,  and 
charcoal.  It  invariably  contains  also  a  small  quantity  of 
moisture.  In  the  following  scheme  of  analysis  all  these  con- 
stituents are  determined  in  a  single  portion  of  the  sample. 

A  light  glass  tube  (a,  fig.  51),  about  10  centimetres  long 


Gunpowder. 


171 


and  i  centimetre  in  diameter,  is  drawn  out  near  the  end  by 
the  aid  of  the  blow-pipe.  The  contracted  portion  should 
measure  about  5  centimetres  long,  and  possess  an  internal 
diameter  of  0*2  centimetre.  At  the  point  where  the  tube  is 
narrowed,  place  a  plug  of  recently-ignited  asbestos,  from  i  -5 
to  2  centimetres  long.  Accurately  weigh  the  tube,  place  in 
it  about  3  grams  of  the  triturated  powder,  and  again 
weigh :  the  increase  of  course  gives  the  amount  taken  for 

FIG.  51. 


analysis.  By  the  aid  of  the  filter-pump  aspirate  a  gentle  cur- 
rent of  air,  dried  by  sulphuric  acid,  through  the  tube  for  10 
or  1 2  hours,  and  again  weigh.  The  loss  indicates  the  amount 
of  moisture. 

The  tube  is  next  fitted  into  a  light  flask  of  about  25  c.c. 
capacity,  provided  with  a  side-tube  in  the  manner  seen  in 
fig.  51.  The  flask  is  accurately  weighed  and  connected  with 
a  tube  of  thin  glass  25  to  30  centimetres  long.  Portions  of 


172  Quantitative  Chemical  Analysis. 

about  3  c.c.  of  carbon  bisulphide  (free  from  moisture,  and 
rectified  by  agitation  with  mercury,  and  redistillation)  are 
poured  over  the  powder.  The  filtrate  running  into  the 
flask  should  be  perfectly  clear.  As  soon  as  the  flask  is 
about  half  filled,  it  is  heated  by  hot  water  at  about  70°. 
The  carbon  bisulphide  distils  over,  and  is  collected  in  a 
dry  test-tube  surrounded  by  cold  water.  The  distillate  is 
again  poured  on  to  the  powder,  and  again  distilled,  the 
operation  being  repeated  6  or  8  times.  After  the  last  distil- 
lation the  residual  sulphur  is  gently  heated,  and  a  current  of 
dry  air  is  drawn  through  the  entire  apparatus  by  attaching 
the  side-tube  to  the  filter-pump.  The  flask  is  re-weighed  : 
the  increase  in  weight  gives  the  amount  of  the  sulphur  in  the 
powder  which  it  is  possible  to  extract  by  means  of  bisulphide 
of  carbon. 

The  tube  containing  the  residual  charcoal  and  nitre, 
together  with  the  minute  quantity  of  sulphur  still  left  in  the 
exhausted  powder,  is  heated  to  100°,  and  a  current  of  air, 
dried  by  sulphuric  acid,  is  drawn  over  it.  The  tube  is  again 
weighed :  its  decrease  in  weight  will  be  slightly  greater  than 
the  increase  in  weight  of  the  flask  containing  the  sulphur. 
The  difference  is  the  amount  of  moisture  which  the  powder 
would  give  up  if  dried  at  100°.  This  slight  difference  is  to 
be  added  to  the  quantity  of  moisture  already  determined. 

About  i  gram  of  the  exhausted  powder  is  shaken  out  into 
a  porcelain  basin,  and  the  tube  is  re-weighed.  The  loss  of 
weight  shows  the  amount  transferred  to  the  basin.  This 
powder  is  then  gently  heated  with  nitric  acid  (perfectly  free 
from  sulphuric  acid)  and  a  few  crystals  of  potassium 
chlorate.  The  liquid  is  evaporated  to  dryness  with  hydro- 
chloric acid,  dissolved  in  water,  filtered  if  necessary,  and 
the  sulphuric  acid  precipitated  in  the  ordinary  manner  by 
barium  chloride.  The  barium  sulphate  is  dried,  ignited,  and 
weighed,  and  the  amount  of  sulphur  equivalent  to  it  is  added 
to  the  main  quantity  extracted  by  the  sulphide  of  carbon. 

The  tube  containing  the  remainder  of  the  exhausted  pow- 


Gunpowder. 


173 


FIG.  52. 


der  is  now  treated  with  water  to  extract  the  nitre.  It 
is  fitted  into  the  bell- 
jar  (fig.  52),  standing  on 
a  plate  of  ground  glass. 
Within  the  jar  and  under- 
neath the  tube  is  a 
weighed  platinum  dish. 
The  side  tube  is  con- 
nected with  the  filter- 
pump.  A  few  cubic 
centimetres  of  cold  water 
are  poured  on  to  the  pow- 
der, and  the  pump  is  set 
in  operation  so  as  to  cause 
the  liquid  to  fall,  drop  by 
drop,  into  the  basin.  To 
avoid  loss  by  splashing, 
the  basin  should  be  as  near  to  the  edge  of  the  tube  as 
possible.  Successive  small  quantities  of  water  of  a  gradually 
increasing  temperature  are  now  poured  over  the  powder, 
water  as  hot  as  possible  being  used  for  the  last  washings. 
50  c.c.  of  water  are  amply  sufficient  to  extract  all  the  nitre 
from  the  residue  of  the  powder,  if  care  be  taken  not  to  use 
too  much  water  for  each  washing.  The  use  of  large  quantities 
of  water  for  washing  is  not  advisable,  since  appreciable 
quantities  of  organic  matter  are  thereby  liable  to  be  dissolved 
out  of  the  charcoal.  The  solution  of  the  nitre  is  evaporated 
to  dryness  in  the  dish,  dried  at  120°,  and  weighed:  the 
weight  is  of  course  calculated  upon  the  entire  quantity  of 
powder.  The  asbestos-plug  is  now  detached  from  the  tube 
by  the  aid  of  a  platinum  wire,  and  the  tube  and  its  contents 
are  again  dried  at  100°,  and  weighed.  It  will  be  generally 
found,  if  the  process  has  been  properly  carried  out,  that  the 
weight  of  the  charcoal  is  slightly  greater  (generally  from  i  to 
2  milligrams)  than  the  amount  calculated  from  the  quantity 
of  nitre  found  :  the  difference  is  due  to  the  fact  that  the  pure 


174  Quantitative  Chemical  Analysis. 

charcoal  retains  water  more  tenaciously,  even  after  drying 
for  some  time  at  100°,  than  when  mixed  with  nitre. 

It  is  sometimes  necessary  to  determine  the  amount  of  car- 
bon, hydrogen,  and  oxygen  in  the  residual  charcoal :  this 
may  readily  be  effected  by  mixing  it,  together  with  the 
asbestos,  with  lead  chromate,  and  burning  it  in  a  stream  of 
oxygen,  and  collecting  the  carbon  dioxide  and  water  in  the 
manner  described  under  organic  analysis.  In  calculating 
the  amount  of  hydrogen,  due  regard  must  be  taken  of  the 
water  retained  by  the  charcoal  after  drying  at  100°. 

III.     LIMESTONES.     HYDRAULIC  MORTAR. 

Limestone  is  essentially  calcium  carbonate,  containing 
more  or  less  magnesium  carbonate,  ferrous  and  manganous 
carbonates  or  oxides,  alumina,  silica,  and  alkalies.  Many 
limestones  also  contain  variable  quantities  of  clay,  sand,  and 
organic  matter,  together  with  chlorine,  fluorine,  phosphoric 
and  sulphuric  acids,  and  iron  pyrites. 

The  method  given  on  p.  85,  which  includes  the  determi- 
nation of  the  essential  constituents,  will  generally  suffice  for 
the  examination  of  limestone  for  technical  purposes,  but 
occasionally  it  is  required  to  estimate  the  substances  which 
are  present  in  smaller  proportion. 

Powder  about  100  grams  of  the  mineral,  mix  uniformly, 
and  dry  at  100°.  Weigh  out  about  2  grams  into  the  flask  A 
(fig.  31),  and  determine  the  amount  of  carbon  dioxide  in  the 
manner  directed  on  p.  86".  Rinse  the  solution  into  a  porce- 
lain basin,  evaporate  to  complete  dryness,  moisten  with  a 
few  drops  of  hydrochloric  acid,  dissolve  in  hot  water,  filter 
through  a  weighed  filter,  wash  the  residue,  and  dry  it  at  100°. 
It  may  consist  of  sand,  clay,  and  separated  silicic  acid,  and 
organic  matter.  The  proportion  of  these  several  substances 
will  be  estimated  hereafter. 

Add  a  few  drops  of  bromine-water  to  the  filtrate,  and  then 
ammonium  chloride  and  a  slight  excess  of  ammonia,  cover 
the  beaker,  and  heat  it  gently  for  some  time.  The  precipi- 


Limestones,  &c.  175 

tare  contains  the  oxides  of  iron,  manganese,  and  aluminium, 
together  with  the  phosphoric  acid ;  it  is  thrown  on  to  a  filter, 
washed  once  or  twice,  re-dissolved  in  a  small  quantity  of 
hydrochloric  acid,  again  mixed  with  bromine-water,  and  re- 
precipitated  with  ammonia,  and  again  filtered.  The  second 
precipitation  effects  the  removal  of  the  small  quantities  of 
lime  and  magnesia  which  are  invariably  thrown  down  with 
the  oxides  on  the  first  precipitation.  The  precipitate  is  well 
washed,  dried,  and  weighed. 

The  lime  and  magnesia  in  the  mixed  nitrate  are  separated 
as  directed  on  p.  88.  The  lime  may  be  estimated  volume- 
trically,  as  described  on  p.  152,  or  it  may  be  weighed  as 
carbonate  or  oxide. 

Determination  of  the  Constituents  present  in  small  quantity. 
—Dissolve  about  50  grams  of  the  mineral  in  dilute  hydro- 
chloric acid  in  a  porcelain  basin,  heat  the  solution  gently  to 
expel  carbon  dioxide,  and  filter  through  a  weighed  filter  into 
a  litre  flask,  wash  the  residue  thoroughly,  dry,  and  weigh  it. 
Dilute  the  filtrate  up  to  the  containing-mark. 

Analysis  of  the  Insoluble  Residue. — (a)  Weigh  out  about 
one-fourth  of  the  insoluble  matter  into  a  platinum  basin,  and 
boil  it  with  strong  solution  of  pure  sodium  carbonate.  Filter, 
and  determine  the  silicic  acid  in  solution  by  acidulation  with 
hydrochloric  acid,  and  evaporation  to  dryness  in  a  platinum 
basin. 

(b)  Weigh  out  another  portion  into  a  platinum  crucible 
and  fuse  with  pure  sodium  and  potassium  carbonates,  extract 
with  hot  water,  acidulate  with  hydrochloric  acid,  evaporate 
to  dryness,  and  separate  the  silica  in  the  usual  manner. 
Deduct  the  amount  of  the    silica  soluble   in   solutions  of 
alkaline  carbonate  (a). 

(c)  A  third  portion  of  the  residue  is  weighed  out  into  a 
platinum  boat  and  heated  in  a  current  of  oxygen  in  a  com- 
bustion-tube   partly  filled   with  copper  oxide,  in  order  to 


176  Quantitative  Chemical  Analysis. 

determine  the  amount  of  organic  matter  (humus).    Accord- 
ing to  Petzholdt  humus  contains  58  per  cent,  of  carbon. 

(d)  Iron  pyrites  is  not  an  infrequent  constituent  of  lime- 
stones :  it  is  found  in  the  insoluble  residue,  after  treatment 
with  dilute  hydrochloric  acid.  To  determine  its  amount  the 
remainder  of  the  insoluble  residue  is  heated  with  nitric  acid 
and  potassium  chlorate,  and  the  proportion  of  the  pyrites 
is  calculated  from  the  quantity  of  sulphuric  acid  obtained. 

Analysis  of  the  Solution. — Transfer  500  c.c.  of  the  liquid 
to  a  porcelain  basin,  evaporate  to  complete  dryness,  and 
heat  the  saline  mass  until  fumes  of  hydrochloric  acid  are  no 
longer  visible.  Moisten  with  strong  hydrochloric  acid,  add 
hot  water,  and  filter  the  solution,  wash  the  residue,  and 
weigh  it  in  a  platinum  crucible.  It  consists  mainly  of  silica, 
but  may  contain  sulphates  of  strontium  and  barium.  Call 
it  Pp.  I. 

To  the  filtrate  add  a  few  drops  of  nitric  acid,  boil,  add 
ammonia,  and  again  boil  until  the  fluid  no  longer  smells  of 
ammonia,  filter,  wash  the  precipitate  once  or  twice,  dissolve 
it  in  hydrochloric  acid,  and  precipitate  again  with  ammonia. 
Call  it  Pp.  II. :  it  contains  ferric  oxide,  alumina,  and  phosphoric 
acid.  The  mixed  filtrates  are  received  in  a  flask  (which  they 
should  nearly  fill),  and  mixed  with  ammonium  sulphide. 
The  flask  is  closed  and  set  aside  for  24  hours  in  a  warm 
place.  The  precipitate  consists  of  manganese  sulphide. 
Filter  it  off  and  wash  it  with  water  containing  ammonium 
sulphide.  Call  it  Pp.  III. 

Treatment  of  the  Precipitates  /.,  //.,  and  ///.—Pp.  I. 
Moisten  the  weighed  precipitate  with  pure  hydrofluoric  acid 
and  a  drop  or  two  of  sulphuric  acid,  and  evaporate  to  dry- 
ness.  Repeat  this  operation,  and  fuse  any  residue  with 
sodium  carbonate,  digest  with  hot  water,  filter,  dissolve  the 
washed  precipitate  in  hydrochloric  acid,  and  add  a  drop  or 
two  of  sulphuric  acid  to  the  solution.  Filter  into  a  small 


Limestones,  &c.  177 

flask  (filtrate  a),  wash  the  precipitate,  and  digest  it  on  the  filter 
for  1 2  or  1 5  hours  with  solution  of  ammonium  carbonate,  the 
tube  of  the  funnel  being  meanwhile  closed  by  a  rod  during  the 
digestion.  Open  the  tube,  wash  the  precipitate  with  water,  and 
treat  it  with  dilute  hydrochloric  acid  (filtrate  b\  again  wash 
with  water,  and  weigh  the  residual  barium  sulphate.  Mix  a 
and  b,  add  ammonia  and  ammonium  carbonate ;  if  a  precipi- 
tate forms  on  standing  it  consists  of  strontium  carbonate ;  it  is 
filtered,  washed  with  ammonia  water,  dried,  and  weighed. 

Pp.  II.  Dissolve  in  hydrochloric  acid  in  a  small  flask,  add 
pure  tartaric  acid,  ammonia,  and  ammonium  sulphide.  Close 
the  flask,  and  after  standing  a  few  hours,  wash  the  iron  sul- 
phide with  water  containing  a  few  drops  of  ammonium 
sulphide.  Dissolve  in  hydrochloric  acid,  add  a  crystal  or 
two  of  potassium  chlorate,  boil,  and  precipitate  with  ammonia, 
and  weigh  the  ferric  oxide. 

To  the  yellow-coloured  filtrate,  containing  the  alumina 
and  phosphoric  acid,  add  a  small  quantity  of  pure  sodium 
carbonate  and  nitre,  evaporate  to  dryness,  and  ignite  until 
the  residue  is  free  from  carbon.  Digest  with  water  once  or 
twice,  pour  off  the  solution  into  a  beaker,  and  treat  the 
residue  with  warm  hydrochloric  acid,  add  the  solution  to 
that  contained  in  the  beaker,  filter  if  necessary,  and  mix  the 
filtrate  with  ammonia.  Filter  off  the  precipitate  and  weigh 
it.  Mix  the  filtrate  with  a  few  drops  of  magnesia  mixture  : 
if  a  precipitate  is  again  formed,  it  consists  of  ammonium 
magnesium  phosphate  :  it  is  washed  with  ammonia  water 
and  weighed  as  magnesium  pyrophosphate.  In  that  case 
the  first  precipitate  has  the  composition  A12P2O8  :  if  no 
precipitate  form,  it  is  probably  a  mixture  of  alumina  and 
aluminium  phosphate.  It  is  redissolved  in  a  small  quantity 
of  hydrochloric  acid  and  mixed  with  molybdic  acid  solution, 
and  treated  as  directed  under  Iron  Ores — Estimation  of 
Phosphorus. 

Pp.  III.  consists  almost  entirely  of  manganese  sulphide. 
It  is  treated  with  moderately  dilute  acetic  acid,  and  the  solu- 

N 


178  Quantitative  Chemical  Analysis. 

tion  is  filtered  if  necessary  ;  if  any  insoluble  matter  remains, 
it  is  tested  for  the  metals  of  Group  III.  The  filtrate  is  heated 
to  boiling,  nearly  neutralised  with  caustic  soda,  and  mixed 
with  bromine  water,  and  the  manganese  dioxide  filtered, 
washed,  dried,  ignited,  and  weighed  as  trimanganic  tetroxide.* 

Determination  of  the  Alkalies.^ — Transfer  300  c.c.  of  the 
solution  to  a  flask,  add  bromine  water,  heat  gently,  and  mix 
with  ammonia  and  ammonium  carbonate.  Allow  the 
liquid  to  stand  for  some  hours,  filter,  wash,  evaporate  the 
filtrate  to  dryness  in  a  platinum  dish,  and  ignite  to  remove 
the  ammonia  salts.  Dissolve  in  water,  boil  with  a  little  milk 
of  lime,  filter,  wash,  remove  the  excess  of  lime  by  ammonium 
carbonate  and  oxalate,  filter,  wash,  and  evaporate  the  filtrate 
to  dryness  ;  again  ignite,  dissolve  in  a  few  drops  of  water, 
filter  once  more  if  necessary,  acidulate  with  hydrochloric 
acid,  and  evaporate  the  solution  of  the  mixed  alkaline 
chlorides  to  dryness  in  a  weighed  platinum  dish.  The 
potassium  and  sodium  are  then  separated  by  means  of  plati- 
num tetrachloride,  as  directed  in  No.  IV.  p.  85. 

Determination  of  Sulphuric  Acid. — To  the  remainder  of 
the  original  solution,  add  one  or  two  drops  of  barium 
chloride,  and  allow  the  liquid  to  stand  for  some  time.  If 
any  precipitate  of  barium  sulphate  is  formed,  filter  it  off, 
wash,  dry,  and  weigh  it. 

*  Chatard  determines  the  small  quantity  of  manganese  present  in  dolo- 
mites, limestones,  &c.,  by  dissolving  the  mineral  in  dilute  nitric  acid, 
boiling  the  solution  with  a  small  quantity  of  lead  peroxide,  filtering 
through  an  asbestos  filter,  and  volumetrically  determining  the  amount  of 
permanganic  acid  in  the  liquid  by  a  dilute  solution  of  ammonium 
oxalate  of  known  strength. 

t  Alkalies  when  present  in  limestones  or  dolomites  may  be  readily 
detected  by  strongly  heating  the  mineral  in  a  platinum  crucible,  boiling 
with  a  little  water,  filtering,  acidulating  with  hydrochloric  acid, 
adding  ammonia  and  ammonium  carbonate,  filtering,  evaporating  the 
filtrate  to  dryness,  and  examining  the  residue  by  means  of  the  specto- 
scope.  The  ammonium  carbonate  precipitate  may  also  be  treated  with 
hydrochloric  acid,  evaporated  to  dryness,  and  examined  for  barium  and 
strontium  in  the  same  manner.  (ENGELBACH.) 


Limestones,  &c.  179 

Determination  of  Chlorine. — Chlorides  are  occasionally 
present  in  dolomites  and  limestones  :  the  amount  of  chlorine 
in  them  may  be  determined  by  dissolving  a  quantity  of  the 
mineral  in  dilute  nitric  acid,  filtering,  and  adding  a  few  drops 
of  silver  nitrate  to  the  filtrate,  and  treating  the  silver  chloride 
as  directed  in  No.  II.  p.  81, 

Determination  of  Fluorine. — A  large  quantity  of  the 
mineral  is  dissolved  in  acetic  acid,  the  solution  is  evaporated 
to  dryness,  and  heated  to  expel  the  excess  of  acetic  acid. 
The  mass  is  repeatedly  treated  with  water,  the  residue  is 
weighed,  and  a  portion  is  tested  for  fluorine.  If  this  substance 
is  found  in  estimable  quantity,  fuse  the  remainder  of  the 
insoluble  residue  with  sodium  carbonate,  boil  with  water, 
filter,  and  wash  with  boiling  water  and  solution  of  ammonium 
carbonate.  Heat  the  filtrate,  which  contains  all  the  fluorine, 
with  an  additional  quantity  of  ammonium  carbonate,  and  after 
some  time  filter  off  the  precipitated  silica  and  alumina.  Boil 
the  filtrate  until  the  ammonium  carbonate  is  completely  ex- 
pelled, and  add  a  few  drops  of  calcium  chloride  solution  ;  if 
any  precipitate  of  calcium  fluoride  forms,  filter  it  off,  wash 
thoroughly  with  hot  water,  dry,  ignite,  and  weigh.  To  ensure 
the  absence  of  calcium  carbonate  in  the  weighed  precipitate, 
digest  it  with  dilute  acetic  acid,  allow  the  precipitate  to 
subside,  pass  the  liquid  through  a  small  filter,  wash  by 
decantation,  dry  the  precipitate  and  filter,  burn  the  latter, 
add  the  ash,  and  again  weigh. 

Determination  of  Water  retained  in  the  Mineral  after  heating 
to  100°. — This  is  effected  in  the  apparatus  seen  in  fig.  53. 
The  weighed  quantity  of  the  mineral  (about  3  grams)  is 
heated  in  the  bulb,  made  of  difficultly -fusible  glass,  in  a 
stream  of  aspirated  air,  dried  by  passing  through  the  first 
calcium-chloride  tube ;  the  water  evolved  is  absorbed  in  the 
second  weighed  tube,  which  also  contains  calcium  chloride. 
The  increase  in  the  weight  of  this  tube  at  the  termination  of 

N  2 


i8o 


Quantitative  Chemical  Analysis. 


the  experiment  gives  the  amount  of  moisture  present  in  the 
mineral.  The  little  flask  contains  strong  sulphuric  acid  ;  it 
serves  to  indicate  the  rate  of  the  current  of  air,  and  prevents 
the  possibility  of  moisture  diffusing  into  the  weighed  calcium- 
chloride  tube. 

CEMENT-STONE,  as  the  material  used  in  the  manufacture 
of  hydraulic  mortar  is  termed,  is  an  impure  limestone  con- 
taining a  considerable  quantity  of  ferrous  carbonate,  alumina, 

\ 

FIG.  53 


and  silica.  Hydraulic  mortar  owes  its  property  of  harden- 
ing under  water  to  the  gradual  formation  of  hydrated  sili- 
cates of  lime  and  alumina,  which  are  very  dense  and  insolu- 
ble in  water. 

IV.    CLAYS. 

Clay  is  a  hydrated  aluminium  silicate,  derived  from  the 
decomposition  of  felspar.  The  purer  varieties  are  perfectly 
white,  and  contain  but  small  quantities  of  lime,  magnesia,  and 


Clay.  181 

oxide  of  iron.  The  red  colour  of  the  common  brick-clay 
after  burning  is  due  to  the  ferric  oxide  which  it  contains. 
The  different  clays  used  in  the  arts  may  be  classed  under 
the  heads  of  slate  clay,  common  clay,  fire-clay,  plastic  clay, 
and  kaolin.  These  varieties  have  essentially  the  same  quali- 
tative composition  ;  their  different  properties  are  mainly 
due  to  the  relative  amounts  of  the  admixed  substances. 
Pure  clay  is  nearly  infusible  ;  but  if  mixed  with  a  sufficient 
amount  of  iron  and  lime,  it  may  be  more  or  less  readily 
melted,  especially  if  free  silica  be  present.  The  ease  with 
which  it  may  be  fused  depends  upon  the  proportion  of  these 
admixtures.  The  following  analyses  will  serve  to  indicate 
the  composition  of  ordinary  fire-clays  : — 

No.  i,  BEST  STOURBRIDGE  CLAY.     No.  2,  INFERIOR  FIRE-CLAY, 
FAULTY  GLASS  POT.    No.  3,  BRICK  CLAY  OF  AVERAGE  QUALITY. 

T.  2.  3. 

Silica      .  73*82  69-91  49  -44 


Alumina 
Ferrous  Oxide 
Lime 
Magnesia 
Alkalies  . 
Water 


15-88  17-44  34-26 

2'95  Fe203     2-89  Fe.Oa     774 

trace  3  -08  I  -48 

trace  4-47  5-14 

0-90  2-21  — 

6-45  1-94 


100-00  100-00 


The  silica  in  clay  exists  partly  as  sand,  partly  as  hydrate, 
and  partly  in  combination  with  bases  ;  the  amount  of  free 
hydrated  silica  seldom  exceeds  i  per  cent.  The  sand  may 
vary  from  15  to  60  per  cent. 

The  porcelain  earth,  or  kaolin  of  the  Chinese,  is  almost 
pure  hydrated  silicate  of  aluminium,  containing  undecom- 
posed  felspar  and  free  silica.  It  has  been  formed  from 
orthoclase  by  the  gradual  abstraction  of  the  whole  of  the 
alkalies,  and  of  about  f  of  the  silica.  When  freed  from  felspar 
and  admixed  silica,  its  average  composition  is  Al2O32SiO2  + 
2H2O,  but  varieties  of  kaolin  are  frequently  found  in  which 
the  relation  of  alumina  to  silica  is  very  different. 

For  many  of  its  applications,  it  is  important  to  know  the 


1 82  Quantitative  Chemical  Analysis. 

relative  proportions  of  coarse  and  fine  sand  present  in  the 
sample,  in  addition  to  the  true  clay.  The  chemical  exami- 
nation, is,  therefore,  usually  preceded  by  a  mechanical 
analysis. 

A.     Mechanical  Analysis. 

This  may  be  effected  with  sufficient  accuracy  by  elutria- 
tion.  The  powdered  clay  is  agitated  in  a  stream  of  water  ; 
the  coarse  particles  subside,  the  finer  particles  are  carried 
away  by  the  current,  and  are  received  in  a  large  beaker,  or 
other  suitable  vessel,  where  they  are  allowed  to  settle.  Of 
the  several  forms  of  apparatus  proposed  for  this  purpose,  the 
simplest  is  that  devised  by  Schulze.  To  a  tall  and  narrow 
glass  (one  of  the  old  forms  of  champagne  glass  does  very 
well)  about  20  centimetres  deep,  and  7  centimetres  in  dia- 
meter at  the  mouth,  is  fastened  a  brass  rim,  about  2  centi- 
metres broad,  carrying  a  short  tube  inclined  downwards. 
A  slow  stream  of  water  is  allowed  to  pass  into  the  glass, 
through  a  funnel-tube  about  40  centimetres  long,  and 
7  millimetres  in  diameter  ;  the  bulb  of  the  funnel  should  be 
about  5  centimetres  in  diameter,  and  its  end  should  be 
drawn  out  until  the  opening  is  only  \\  millimetre  in 
diameter.  Triturate  30  or  40  grams  of  the  air-dried  clay  in 
a  mortar,  transfer  it  to  a  porcelain  dish,  and  boil  it  with 
about  70  c.c.  of  water  for  thirty  minutes.  Repeatedly  crush 
the  sedimentary  matter  with  a  pestle,  and  agitate  the  liquid 
so  as  to  disintegrate  the  clay  completely.  Allow  to  cool, 
and  transfer  the  contents  of  the  dish  to  the  narrow  glass  ; 
suspend  the  tube-funnel  within  the  glass,  so  that  its  end  is 
about  2  or  3  millimetres  from  the  bottom,  and  so  regulate 
the  stream  of  water  that  the  funnel  is  kept  constantly  half- 
filled  with  water.  The  finer  particles  are  stirred  up,  and  are 
carried  away  in  the  current  of  water,  through  the  lateral 
tube,  into  the  beaker  placed  to  receive  it ;  the  coarse  sand 
remains  in  the  elutriating  glass.  The  flow  of  the  water  is 
arrested  when  it  runs  away  nearly  clear,  and  the  liquid  still 


Clay.  183 

in  the  glass  is  decanted  into  the  beaker.  The  coarse  sand 
is  washed  out  into  a  porcelain  crucible,  dried,  ignited,  and 
weighed. 

In  about  six  hours  the  liquid  in  the  beaker  will  be  nearly 
clear  ;  the  whole  of  the  fine  sand  will  certainly  have  been 
deposited  by  that  time.  The  supernatant  liquid  is  poured 
away :  the  deposit  is  rinsed  back  into  the  narrow  glass,  and 
the  process  of  elutriation  is  repeated.  The  flow  of  the  water  is 
so  regulated  that  its  level  in  the  funnel-tube  is  about  3  centi- 
metres higher  than  that  in  the  glass ;  in  about  four  hours  the 
whole  of  the  clay  proper  will  have  been  carried  away  through 
the  discharge-pipe.  The  residual  fine  sand  is  then  rinsed  into 
a  weighed  porcelain  crucible,  dried,  ignited,  and  weighed. 

The  water  in  the  air-dried  sample  is  determined  by  igni- 
ting a  second  portion  for  some  time  ;  the  amount  of  the  clay 
proDer  is  determined  by  difference. 

B.     Chemical  Analysis. 

The  air-dried  clay  is  powdered  as  finely  as  possible,  a 
portion  is  weighed  out  into  a  large  porcelain  crucible,  and 
heated  in  the  steam-chamber  for  several  days.  The  moisture 
is  calculated  from  the  loss. 

About  2  grams  of  the  dried  powder  are  then  heated  in  a 
platinum  dish  with  excess  of  moderately-concentrated  sul- 
phuric acid  for  about  8  or  10  hours,  and  the  mass  is 
evaporated  to  dryness  to  expel  the  acid.  When  cold,  the 
residue  is  boiled  with  water,  the  solution  is  filtered,  and  the 
insoluble  matter,  consisting  of  the  silica  originally  existing  in 
union  with  the  bases  in  the  clay,  and  also  of  the  small 
quantity  of  free  silica,  together  with  the  sand,  is  washed, 
dried,  and  weighed.  It  is  then  repeatedly  boiled  with  solu- 
tion of  sodium  carbonate  in  the  platinum  dish,  the  liquid 
filtered,  and  the  residual  sand  washed  with  hot  water,  next 
with  water  slightly  acidified  with  hydrochloric  acid,  and 
finally  with  pure  water.  It  is  then  dried  and  weighed.  Its 
weight,  subtracted  from  the  total  weight  of  the  residue  left 


1 84  Quantitative  Chemical  Analysis. 

after  treatment  with  sulphuric  acid,  gives  the  amount  of  true 
silicic  acid. 

The  quantity  of  uncombined  silicic  acid  in  the  clay  may 
be  determined  by  boiling  a  weighed  quantity  of  the  sample 
dried  at  100°  with  a  strong  solution  of  sodium  carbonate, 
filtering  the  liquid,  and  evaporating  to  dryness  with  excess 
of  hydrochloric  acid.  The  separated  silicic  acid  is  washed, 
dried,  and  weighed.  Titanium  dioxide  is  not  an  infrequent 
constituent  of  clays.  It  may  be  detected  in  the  residue  after 
treatment  with  sulphuric  acid  :  an  aliquot  portion  is  heated 
with  hydrofluoric  and  sulphuric  acids,  whereby  the  greater 
portion  of  the  silica  is  volatilised  :  the  residue  is  fused  with 
acid  sulphate  of  potassium.  Dissolve  in  cold  water,  filter  if 
necessary,  and  boil.  The  titanium  dioxide  isreprecipitated: 
it  is  to  be  washed,  dried,  and  weighed.  To  the  filtrate 
obtained  after  treatment  with  sulphuric  acid,  and  containing 
the  bases,  add  a  slight  excess  of  lead  nitrate  solution,  and 
allow  the  turbid  liquid  to  stand  for  a  few  hours.  Filter, 
wash  the  lead  sulphate,  adding  the  washings  to  the  filtrate, 
and  pass  sulphuretted  hydrogen  through  the  liquid  to  remove 
the  excess  of  lead,  again  filter,  and  evaporate  the  solution  to 
dryness.  The  alumina,  iron,  lime,  magnesia,  and  alkalies 
are  obtained  as  nitrates.  Heat  gradually  to  about  250°  until 
no  more  fumes  of  nitric  acid  are  evolved.  The  residue 
consists  of  alumina  and  ferric  oxide,  calcium,  magnesium, 
and  alkaline  nitrates.  Moisten  the  residue  with  a  concen- 
trated solution  of  ammonium  nitrate,  and  heat  on  the  water- 
bath  until  no  further  evolution  of  ammonia  is  perceived. 
Add  hot  water,  filter,  wash  the  alumina  and  ferric  oxide, 
dry,  ignite,  and  weigh.  Transfer  the  weighed  oxides  to  a 
porcelain  boat,  and  heat  in  a  stream  of  dry  hydrogen  or  coal- 
gas  for  half  an  hour.  Treat  the  mixture  of  alumina  and 
reduced  iron  with  very  dilute  nitric  acid  (i  pt.  acid  to  40  of 
water),  warm,  filter,  and  precipitate  the  ferric  oxide  by 
ammonia,  wash,  dry,  and  weigh  it.  The  mixture  may  also 
be  heated  with  dilute  sulphuric  acid,  filtered,  reduced  with 


Manganese  Ores.  185 

a  small  piece  of  zinc,  and  titrated  with  a  weak  solution 
of  potassium  permanganate.  Deduct  the  weight  of  ferric 
oxide  from  that  of  the  mixed  oxides :  the  difference  gives 
the  quantity  of  alumina. 

To  the  nitrate  containing  the  nitrates  add  ammonium 
oxalate  and  weigh  the  precipitate  as  caustic  lime.  Evaporate 
the  nitrate  to  dryness,  ignite  to  expel  the  ammonium  salts, 
add  excess  of  oxalic  acid  (sufficient  to  convert  all  the  bases 
present,  considered  as  potash,  into  quadroxalates),  treat  with 
a  small  quantity  of  water,  and  again  evaporate  to  dryness. 
Ignite  gently :  the  magnesium  oxalate  is  converted  into 
magnesia,  and  the  alkaline  oxalates  into  carbonates.  Treat 
repeatedly  with  small  quantities  of  water,  filter,  dry,  and  weigh 
the  magnesia. 

Add  a  few  drops  of  hydrochloric  acid  to  the  filtrate, 
evaporate  to  dryness,  ignite  gently,  and  weigh  the  alkaline 
chlorides.  The  potash  and  soda  may  be  separated  by  platinum 
tetrachloride  (see  p.  84),  or  their  proportion  may  be  deter- 
mined by  a  dilute  standard  silver  solution  in  the  manner 
described  on  pt  127. 

The  magnesia  may  also  be  precipitated  by  sodium  phos- 
phate, and  the  alkalies  determined  in  a  separate  portion  by 
ignition  with  calcium  carbonate  and  ammonium  chloride 
(see  Glass,  p.  99). 

ASSAY  OF  MANGANESE   ORES  (PYROLUSITE, 
BRAUNITE,  &c.). 

Of  the  many  methods  which  have  been  proposed  for  the 
valuation  of  these  substances,  those  of  Bunsen  and  of 
Fresenius  and  Will  are  on  the  whole  the  most  accurate 
and  convenient.  As  the  value  of  the  oxide  depends  upon 
the  amount  of  chlorine  which  it  yields  on  heating  with 
hydrochloric  acid,  the  method  of  Bunsen  is  perhaps  the 
more  generally  applicable,  since  it  directly  determines  the 
amount  of  chlorine  thus  evolved.  It  has  the  advantage  too 
of  being  rapidly  carried  out,  and  is  therefore  well  adapted 


1 86  Quantitative  Chemical  Analysis. 

to  the  requirements  of  manufacturing  establishments.  When 
manganese  dioxide  is  brought  into  contact  with  hydrochloric 
acid,  the  following  reaction  occurs  : 

MnO2  +  4HC1     =     MnCl2  +   C12  +   2H2O. 

70*92  parts  of  chlorine  or  2537  parts  of  iodine  are  equiva- 
lent to  86*04  parts  of  manganese  dioxide  (Mn  =  54*04, 
Schneider).  If  the  chlorine  be  led  into  a  solution  of 
potassium  iodide,  an  equivalent  quantity  of  iodine  is  libe- 
rated ;  this  may  be  determined  by  the  method  given  on  p.  164. 

Weigh  out  0*5  gram  of  the  finely-powdered  ore  into  the 
little  bulb  a  (fig.  50),  add  hydrochloric  acid,  and  drive  over  the 
chlorine  into  the  U-tube,  in  which  you  have  previously 
placed  25  cubic  centimetres  of  the  strong  solution  of  potas- 
sium iodide. 

The  remainder  of  the  operation  is  conducted  as  described 
on  p.  163. 

V.     Gravimetrical  Method  of  Fresemus  and  Will. 

This  process  depends  upon  the  action  of  manganese 
dioxide  on  oxalic  acid,  in  presence  of  sulphuric  acid ;  when 
these  substances  are  brought  together,  the  manganese  dioxide 
parts  with  an  atom  of  its  oxygen  to  the  oxalic  acid,  which  is 
thereby  completely  converted  into  carbon  dioxide  and  water, 
and  the  manganese  protoxide  combines  with  sulphuric  acid 
to  form  manganous  sulphate  ;  thus  : — 

Mn02  +  C202(OH)2  +  H2S04  =  MnSO4  +  2CO2  +  2H2O. 

88  parts  of  carbon  dioxide  evolved,  correspond  therefore  to 
86*04  parts  of  manganese  dioxide. 

The  decomposition  may  be  effected  in  the  apparatus 
represented  in  fig.  31.  From  2  to  4  grams  of  the  finely- 
powdered  ore,  according  to  its  supposed  richness,  are 
weighed  out  into  the  flask  A,  and  covered  with  dilute  sul- 
phuric acid.  A  strong  solution  of  oxalic  acid  is  then  allowed 
to  flow  from  the  bulb-tube,  and  the  evolved  carbonic  acid 


Manganese  Ores.  187 

collected  in  the  weighed  soda-lime  tube  c.  It  is  advisable  to 
place  a  weighed  potash-apparatus  (Geissler's  form  is  the 
most  convenient)  before  the  U-tube. 

Certain  ores  of  manganese  contain  considerable  quantities 
of  earthy  carbonates  ;  on  treatment  with  an  acid  their  car- 
bonic acid  is  of  course  liberated,  and  thus  tends  to  in- 
crease the  apparent  value  of  the  sample.  By  means  of 
the  apparatus  above  mentioned,  this  amount  of  carbonic 
acid  may  be  readily  determined  ;  it  is  only  necessary  to 
re-weigh  the  absorption  tubes  before  the  addition  of  the 
oxalic  acid.  Of  course,  the  precaution  must  be  taken  to 
aspirate  air  through  the  appa- 
ratus before  disconnecting 
the  several  parts. 

The  original  apparatus  de- 
vised by  Fresenius  and  Will 
is  represented  in  fig.  54.  It 
consists  of  two  flasks,  a  and  £, 
connected  together  by  a  thin 
glass  tube  d^  one  end  of  which 
terminates  just  below  the 
cork  of  a,  the  other  ends  a 
few  millimetres  above  the 
bottom  of  b.  The  thin  tube  c  passes  down  to  the  bottom 
of  a  -,  the  short  tube  e  ends  just  below  the  cork  of  b.  The 
two  flasks  are  of  about  100  cubic  centimetres  capacity  ;  they 
are  made  of  thin  glass,  so  as  to  weigh  as  little  as  possible. 
Weigh  out  into  a  from  3  to  5  grams  of  the  powdered 
sample,  add  from  5  to  6  grams  of  sodium  oxalate,  and  half 
fill  b  with  strong  sulphuric  acid.  Weigh  the  entire  apparatus. 
Place  a  short  piece  of  caoutchouc  tubing  over  c9  and  close 
it  by  a  glass  rod,  slip  a  longer  piece  of  caoutchouc  tubing 
over  <?,  and  aspirate  two  or  three  bubbles  of  air  from  a 
through  the  sulphuric  acid.  On  discontinuing  the  suction, 
the  acid  rises  in  the  tube  ;  if  the  column  remains  stationary, 
the  apparatus  is  air- tight.  Now  aspirate  more  air  from  a9 


1 88  Quantitative  Chemical  Analysis. 

so  as  to  cause  a  small  quantity  of  sulphuric  acid  to  pass 
over  from  b.  Carbonic  acid  is  immediately  evolved,  and  is 
dried  by  passing  through  the  sulphuric  acid  in  b.  When  the 
evolution  of  gas  begins  to  slacken,  draw  over  in  the  same 
manner  fresh  quantities  of  the  acid.  The  complete  decom- 
position of  the  ore  requires  from  five  to  ten  minutes  ;  the  ter- 
mination of  the  reaction  is  indicated  by  the  appearance  of 
the  residue  in  #,  which  will  no  longer  be  black,  and  also  from 
the  non-evolution  of  carbonic  acid  when  a  fresh  quantity  of 
sulphuric  acid  flows  over  into  a.  Remove  the  glass  stopper 
from  c,  and  aspirate  a  slow  current  of  air,  dried  by  sulphuric 
acid,  through  the  apparatus  for  five  minutes.  When  the 
flask  is  cold,  remove  the  caoutchouc  tubing,  re-weigh,  again 
aspirate  dry  air  through  it,  and  again  weigh.  The  two 
weighings  ought  to  agree.  The  loss  of  weight  indicates  the 
amount  of  carbon  dioxide  evolved  by  the  action  of  the  man- 
ganese dioxide  upon  the  oxalic  acid. 

In  the  case  of  ores  containing  carbonates,  treat  the 
weighed  portion  in  a  with  a  little  water,  add  two  or  three 
drops  of  dilute  sulphuric  acid,  and  heat  on  the  water-bath. 
In  about  ten  or  fifteen  minutes  test  the  liquid  with  a  slip  of 
blue  litmus  paper ;  if  it  is  not  acid,  add  a  few  more  drops 
of  dilute  sulphuric  acid,  and  continue  the  heating.  When 
the  carbonates  are  completely  decomposed,  neutralise  with 
caustic  soda,  free  from  carbonic  acid,  allow  the  liquid  to 
cool,  add  the  sodium  oxalate,  connect  the  flasks  together, 
and  proceed  as  above. 

With  proper   care    this    method  gives   very  concordant 
FIG.  S4A.       results ;   two   analyses   should   not  differ  by 
more  than  0-25  per  cent. 

The  great  weight  of  the  apparatus,  and  the 
surface  it  exposes,  are  its  chief  disadvantages. 
To  obviate  these  sources  of  error,  various 
modified  forms  of  it  have  been  devised  ;  one 
of  the  most  convenient  of  these  is  seen  in 
fig-  54A. 


Manganese  Ores.  189 

VI.      Volumetric  Determination  by  means  of  Iron  and 
Potassium  Permanganate  Solution. 

About  i '5  to  2  grams  of  iron  wire,  perfectly  free  from 
rust,  are  accurately  weighed  out,  and  dissolved  in  100  c.c.  of 
dilute  sulphuric  acid  (i  of  acid  to  4  of  water),  in  the  appa- 
ratus represented  in  fig.  48,  p.  149.  About  the  same 
weight  of  the  finely-powdered  ore  to  be  tested  is  added  to 
the  solution  of  iron,  and  the  liquid  is  heated  gently,  until 
the  whole  of  the  manganese  is  dissolved.  The  solution  is 
boiled,  the  water  allowed  to  recede  (see  p.  148),  and  the 
contents  of  the  flask  made  up  to  250  c.c.  When  cold  the 
amount  of  residual  ferrous  sulphate  is  determined  by  per- 
manganate solution  : 

2FeSO4   +  MnO2  +   2H2SO4     =     Fe2(SO4)3  + 
MnSO4  +   2H2O. 

112  parts  of  iron  correspond  to  86 '04  of  manganese  dioxide. 
Example. — Weighed  out  1*562  grams  of  iron  wire. 
1-562  x  -996  =  1*556  pure  iron.  Dissolved  in  the 
requisite  quantity  of  dilute  sulphuric  acid,  and  added  1*285 
gram  of  manganese  dioxide.  When  cold,  the  solution 
required  20-2  c.c.  of  permanganate  solution;  i  c.c.  perman- 
ganate =  0*01096  Fe.  Accordingly,  20*2  x  0*01096  = 
0*2215  Fe.  J '556  — 0*2215  =  z'3345  gram  of  iron  has 
been  oxidised  by  the  manganese  dioxide. 

112    \    86*04     I"      1'3345    •    x'        x  —   r'O25 
and  1*285    •     I<025     •'•      I0°    •    79'8o. 

The  ore,  therefore,  contained  79*80  per  cent,  of  manga- 
nese dioxide. 

VII.      Volumetric  Determination  by  means  of  Oxalic  Acid 
and  Potassium  Permanganate. 

Since  the  amount  of  oxalic  acid  in  solution  can  be  readily 
determined  by  means  of  potassium  permanganate  (see  p.  151), 
the  reaction  between  the  manganic  oxide,  oxalic  acid,  and 


IQO  Quantitative  Chemical  Analysis. 

sulphuric  acid  may  be  made  the  basis  of  a  volumetric 
method.  It  is  merely  necessary  to  heat  the  finely-powdered 
ore  with  an  excess  of  oxalic  acid,  and  on  the  completion  of  the 
decomposition  to  determine  the  amount  remaining  in  the 
liquid  by  means  of  standard  potassium  permanganate  solution. 
About  2  grams  of  the  ore  are  weighed  out  into  a  flask,  and 
gently  heated  with  50  cubic  centimetres  of  normal  oxalic  acid 
solution,  and  5  or  6  cubic  centimetres  of  strong  sulphuric  acid  : 
when  the  decomposition  is  at  an  end  (which  may  be  known 
by  the  absence  of  any  black  grains  in  the  sediment),  the 
solution  is  filtered  into  a  J-litre  flask,  the  sediment  and  filter 
washed,  and  the  liquid  diluted  to  250  cubic  centimetres. 
After  shaking,  100  cubic  centimetres  of  the  liquid  are  intro- 
duced into  a  beaker,  and  the  solution  titrated  by  potassium 
permanganate  (see  p.  151).  The  determination  is  repeated 
with  a  second  portion  of  100  cubic  centimetres.  The  mean  of 
the  two  results  multiplied  by  2-5  gives  the  amount  of  residual 
oxalic  acid,  and  this,  subtracted  from  the  amount  originally 
taken,  shows  the  quantity  of  acid  decomposed  by  the  man- 
ganese. From  the  equation 

MnO2  +  C2O2(OH)2     =     2COi  +   MnO  +  H2O 

it  is  seen  that  90  parts  of  oxalic  acid  correspond  to  86-04  of 
manganese  dioxide. 

Many  samples  of  manganese  ore  contain  more  or  less 
ferrous  oxide,  which  becomes  oxidised  at  the  expense  of  a 
portion  of  the  chlorine  evolved  on  treating  the  mixture  with 
hydrochloric  acid.  The  method  of  Fresenius  and  Will,  and 
the  volumetric  modification  above  described,  are  therefore  apt 
to  assign  too  high  a  value  to  the  manganese,  the  quality  of 
which,  as  we  have  already  remarked,  depends  solely  upon 
the  quantity  of  available  chlorine  which  it  can  liberate. 

VIII.     Determination  of  Moisture  in  Manganese  Ores. 

All  manganese  ores  contain  variable  amounts  of  moisture, 
the  exact  determination  of  which  is  a  point  of  some 


Manganese  Ores.  IQI 

importance.  By  repeated  experiments  it  has  been  found  that 
a  temperature  of  120°  maintained  for  about  an  hour  and 
a  half  is  sufficient  to  expel  the  whole  of  the  hygro- 
scopic moisture,  without  eliminating  any  of  the  true  water  of 
hydration.  A  few  grams  of  the  powdered  oxide  are  intro- 
duced into  the  weighed  tube,  fig.  29,  and  heated  to  120° 
for  about  an  hour  and  a  half.  The  loss  of  weight  gives  the 
amount  of  moisture.  If  it  is  preferred  to  dry  the  sample  at 
1 00°  the  heat  must  be  maintained  for  at  least  six  hours 
before  a  constant  weight  will  be  obtained. 

The  dried  and  finely-powdered  oxide  is  exceedingly  hy- 
groscopic, and  if  the  sample  is  dried  before  being  analysed 
there  is  great  risk  that  in  the  operations  of  weighing  the 
portion  will  take  up  fresh  quantities  of  moisture.  It  is 
better  to  keep  the  powdered  sample  undried,  in  a  well- 
corked  test-tube,  and  to  weigh  out  portions  for  the  deter- 
mination of  the  oxide  and  water  at  the  same  time,  and  to 
conduct  the  two  operations  simultaneously.  The  percentage 
amount  of  oxide  in  the  sample  when  dried  may  be  after- 
wards readily  calculated. 

IX.     Determination  of  the  Amount  of  Hydrochloric  Add 
required  to  decompose  Manganese  Ore. 

Two  samples  of  manganese  ore  may  show  on  analysis  the 
same  amount  of  available  dioxide — that  is,  may  liberate  the 
same  amount  of  free  chlorine  and  yet  require  very  different 
amounts  of  hydrochloric  acid  to  effect  their  complete  de- 
composition. The  elimination  of  a  given  quantity  of  chlorine 
by  means  of  hausmannite,  for  example,  requires  the  expendi- 
ture of  twice  the  amount  of  hydrochloric  acid  needed  to 
yield  the  same  quantity  from  the  binoxide  : 

Mn3O4  +  8HC1     =     3MnCl2  +   C12  +  4H2O. 
Mn02    +  4HC1     =     MnCl2     +   C12  +   2H2O. 

Moreover,  since  most  ores  of  manganese  contain  carbonates, 
and  gangue,  decomposable  by  hydrochloric  acid,  it  is  always 


192  Quantitative  Chemical  Analysis. 

necessary  to  employ  a  larger  quantity  of  acid  than  corresponds 
to  the  amount  of  available  binoxide  found.  Accordingly  it 
is  often  required  to  determine  exactly  the  quantity  of  acid 
needed  to  effect  the  complete  decomposition  of  the  ore. 

Dissolve  a  quantity  of  recrystallised  copper  sulphate  in 
warm  water,  and  carefully  add  solution  of  ammonia,  witVi 
constant  stirring,  until  the  bluish-green  precipitate  is  very 
nearly  dissolved.  If  the  exact  point  is  overstepped,  add  a 
little  more  solution  of  copper  sulphate  until  the  precipitate 
just  reappears.  Filter  the  solution,  and  determine  its  value 
by  withdrawing  10  cubic  centimetres,  and  adding  standard 
sulphuric  acid  until  the  liquid  remains  permanently  turbid. 
Now  weigh  off  about  i  gram  of  the  powdered  manganese 
ore  into  a  small  flask  fitted  with  a  cork,  in  which  is  fixed 
an  obtusely -bent  tube  about  3  feet  in  length,  and  add  to  it 
10  cubic  centimetres  of  moderately-concentrated  hydrochloric 
acid  (sp.  gr.  ri),  and  heat  gently  ;  the  bent  tube  is  to 
be  so  arranged  that  the  condensed  water  flows  back 
into  the  flask.  Whilst  the  liquid  is  heating,  measure  off  a 
second  quantity  of  10  cubic  centimetres  of  the  acid,  run  it 
into  a  beaker,  and  add  the  standardised  ammonio-copper 
solution,  with  .constant  stirring,  until  the  liquid  is  just 
rendered  turbid.  Heat  the  contents  of  the  flask  more 
strongly  for  a  few  minutes  to  expel  the  chlorine;  let  the 
flask  cool,  add  a  quantity  of  cold  water  to  it,  throw  the  solu- 
tion on  to  a  filter,  wash,  and  again  titrate  with  the  copper 
solution.  The  difference  expresses  the  amount  of  hydro- 
chloric acid  required  to  decompose  the  ore.  As  the  amount 
of  chlorine  evolved  is  known  from  a  determination  made  by 
one  of  the  preceding  methods,  the  quantity  remaining  in  com- 
bination as  manganous  chloride,  &c.,  is  readily  calculated. 
This  method,  although  not  absolutely  exact,  affords  results  of 
sufficient  accuracy  for  technical  purposes.  The  ordinary  acidi- 
metric  methods  are  here  inapplicable,  since  the  manganous 
chloride  possesses  an  acid  reaction  which  interferes  with  the 
process.  This  solution  (known  as  Kieffer's)  is  frequently  of 


Bleaching  Powder.  193 

service  in  testing  liquors  containing  free  acid  or  salts 
which  redden  litmus  ;  in  determining,  for  example,  the 
amount  of  free  acid  in  liquids  from  galvanic  batteries, 
&c.  It  may  be  also  used  in  determining  the  strength  of 
vinegars,  as  the  brown  colour  of  the  solution  in  no  way 
interferes  with  the  completion  of  the  reaction.  It  is  neces- 
sary from  time  to  time  to  redetermine  the  strength  of  the 
solution  by  standard  acid,  as  it  experiences  slight  alteration 
on  keeping. 

X.     BLEACHING  POWDER. 

Bleaching  powder  or  chloride  of  lime  is  formed  by  the 
action  of  chlorine  upon  calcium  hydrate.  The  relation  of 
its  constituents  in  the  freshly-prepared  substance  is  repre- 
sented by  the  formula  Ca3H6O6.Cl4.  When  allowed  to  stand 
in  contact  with  air  and  light,  chloride  of  lime  suffers 
decomposition,  and,  after  treatment  with  water,  the  calcium 
chloride  is  found  to  have  increased  in  quantity,  whilst  the 
hypochlorite  has  suffered  a  corresponding  diminution. 
When  exposed  to  moist  air  containing  carbonic  acid, 
bleaching  powder  is  decomposed,  hypochlorous  acid  is 
evolved,  and  calcium  carbonate  formed.  When,  there- 
fore, chloride  of  lime  is  used  as  a  disinfectant,  the  active 
agent  in  ordinary  circumstances  is  hypochlorous  acid,  and 
not  free  chlorine,  as  formerly  supposed.  At  a  moderate 
temperature  (50°)  dry  chloride  of  lime  is  converted  into  cal- 
cium chlorate,  and  the  mass  becomes  pasty  from  the 
separation  of  water : 

3Ca3H606Cl4  =  sCaC!2  +  Ca(ClO3)2  +  3CaH2O2  +  6H2O. 

This  change  goes  on  at  a  diminished  rate  even  in  direct 
sunlight.  Chloride  of  lime  is  decomposed  by  water,  calcium 
hydrate  separates  out,  and  calcium  chloride  and  hypo- 
chlorite pass  into  solution  : 

Ca3H606Cl4  =  CaH202  +  CaCl2  +  Ca(ClO)2  +  2H2O. 
It  is  highly  probable   that  the  hypochlorite  thus  formed 


194  Quantitative  Chemical  Analysis. 

is  only  produced  by  the  action  of  the  water,  and  does  not 
exist  pre-formed  in  the  bleaching  powder. 

Since  the  value  of  the  commercial  article  depends  entirely 
upon  the  amount  of  hypochlorous  acid  which  it  can  produce, 
and  since  the  circumstances  of  heat,  moisture,  air,  and  light 
exercise  such  an  important  influence  upon  the  proper  pro- 
duction and  stability  of  the  bleaching  powder,  it  is  evident 
that,  as  manufactured  and  stored,  it  must -vary  very  consider- 
ably in  quality.  The  most  concentrated  preparation  which 
can  be  obtained  by  saturating  calcium  hydrate  with  chlorine, 
contains  about  38-5  per  cent,  of  available  chlorine,  but  the 
great  bulk  of  the  substance  found  in  commerce  rarely  contains 
more  than  from  32  to  37  per  cent,  of  which  i  or  2  per  cent, 
is  without  bleaching  power,  being  present  in  the  form  of  cal- 
cium chlorate.  In  badly-made  bleaching  powder  the  amount 
of  chlorate  present  is  occasionally  equal  to  8  or  10  per  cent, 
of  available  chlorine — nearly  one-fourth  of  the  amount 
which  ought  to  be  contained  in  the  product.  Many 
methods  have  been  proposed  to  estimate  the  available 
chlorine  present  in  bleaching  powder,  the  majority  being 
based  on  the  oxidising  effect  of  the  hypochlorites,  but  a 
great  number  are  inaccurate,  in  that  they  do  not  take  cogni- 
sance of  the  presence  of  this  admixed  chlorate,  which,  under 
the  circumstances  of  the  valuation-processes,  reacts  like 
chlorine,  although  it  has  no  bleaching  effect. 

The  best  and  most  convenient  chlorimetrical  methods 
hitherto  proposed  are  those  of  Penot  and  Bunsen. 

Penofs  Method. — This  process  is  based  upon  the  conversion 
of  an  alkaline  arsenite,  by  the  chloride  of  lime  solution,  into 
an  arseniate : 

As2O3  +  Ca(ClO)2     =     As2O5  +  CaCl2. 

The  final  point  of  the  reaction  is  determined  by  means  of 
potassium  iodide  and  starch  ;  so  long  as  any  hypochlorite 
remains  undecomposed,  a  drop  of  the  solution  brought  into 
contact  with  potassium  iodide  and  starch  renders  that  mixture 


Bleaching  Powder.  195 

blue.  This  mixture  of  iodide  and  starch  is  conveniently 
employed  in  the  form  of  test-papers.  3  grams  of  arrowroot, 
potato,  or  wheat  starch  are  rubbed  into  a  thin  cream  with 
50  or  60  cubic  centimetres  of  warm  water.  Pour  the  mixture 
into  about  200  cubic  centimetres  of  water,  and  heat  the 
liquid,  with  constant  stirring,  until  it  boils :  now  add  i  gram 
of  potassium  iodide  and  i  gram  of  pure  carbonate  of  soda 
dissolved  in  a  little  water,  and  dilute  the  mixture  to  500  cubic 
centimetres.  Moisten  a  number  of  strips  of  Swedish  filter- 
paper,  or  other  unsized  paper  of  good  quality,  with  the 
solution,  and  when  dry,  preserve  them  in  a  wide-mouthed 
stoppered  bottle.  To  prepare  the  arsenious  acid  solution, 
powder  a  quantity  of  the  purest  sublimed  arsenious  acid  (free 
from  arsenic  sulphide),  and  weigh  off  exactly  4-95  grams  into 
a  litre  flask,  add  about  25  grams  of  recrystallised  sodium  car- 
bonate (free  from  sodium  sulphide,  sulphite,  or  thiosulphate) 
and  200  cubic  centimetres  of  water.  Boil  the  solution  gently, 
and  shake  it  continually  until  the  arsenious  acid  is  dissolved: 
when  the  solution  is  cold  dilute  it  exactly  to  one  litre.  This 
constitutes  a  deci-normal  solution  of  arsenious  acid :  the 
equivalent  of  As2O3  is  1 98.  i  eq.  can  take  up  2  atoms  of  oxygen 
to  form  As2O5,  or  is  equivalent  to  4  of  Cl.  Since  it  is  diffi- 
cult to  weigh  out  exactly  the  required  quantity  of  arsenious 
acid,  it  is  preferable  to  take  a  round  number,  about  5  grams, 
and  dilute  proportionally. 

Example. — 5*013  grams  were  weighed  out  into  the  litre 
flask,  25  grams  of  sodium  carbonate  and  200  cubic  centi- 
metres of  water  added  :  after  complete  solution  and  cooling 
the  liquid  was  diluted  to  i  litre,  and  127  cubic  centimetres 
of  water  were  added  by  means  of  a  burette  ;  since 

4-95   :   1000    ::    5*013   :   10127. 

The  solution  in  the  flask  is  well  shaken,  and  decanted  off 
into  a  number  of  small  well-stoppered  bottles  :  this  precaution 
diminishes  the  liability  of  the  solution  to  change  on  exposure 
to  the  air.  If  the  solution  is  perfectly  free  from  sodium 

o  2 


196  Quantitative  Chemical  Analysis. 

thiosulphate,  or  sulphite,  sodiuin  or  arsenic  sulphides,  there 
is  far  less  chance  of  it  suffering  alteration.* 

The  sample  of  bleaching  powder  to  be  tested  is  well 
mixed,  and  about  10  grams  are  weighed  out  into  a  porcelain 
mortar ;  50  or  60  cubic  centimetres  of  water  are  added,  and 
the  mixture  is  rubbed  to  a  thin  cream  ;  it  is  allowed  to  settle 
for  a  few  minutes,  and  the  supernatant  liquid  (which  is  still 
turbid)  poured  into  a  litre  flask.  The  sediment  in  the  mortar 
is  triturated  with  fresh  water,  and  the  operation  is  repeated 
until  the  whole  of  the  chloride  of  lime  has  been  brought 
into  the  litre  flask.  Fill  up  to  the  mark,  and  shake.  Have 
the  burette  ready  filled  to  the  zero  mark,  withdraw  50  cubic 
centimetres  of  the  turbid  solution,  run  it  into  a  beaker,  and 
add  the  arsenious  acid  solution,  with  constant  stirring,  until 
a-  drop  from  the  beaker,  taken  out  on  a  glass  rod,  and  brought 
into  contact  with  a  strip  of  the  iodised  paper  moistened 
with  water  on  a  white  plate,  no  longer  gives  a  blue  stain. 
There  is  no  difficulty  in  hitting  the  final  point ;  the  gradually 
increasing  faintness  in  the  blue  colour  of  the  drops  indicates 
with  great  accuracy  the  progress  of  the  reaction.  In  making 
a  second  determination,  care  must  be  taken  to  shake  the 
contents  of  the  litre  flask  before  withdrawing  the  solution ; 
if  this  precaution  be  neglected,  the  second  determination 
will  give  a  much  lower  result — a  difference  of  2  or  3  cubic 
centimetres  being  not  unfrequently  obtained  in  testing  the 
clear  and  the  turbid  liquids. 

Example. — 10*99  grams  of  bleaching  powder  were  treated 
as  directed,  and  diluted  to  i  litre.  50  cubic  centimetres  of 

*  When  a  great  number  of  chlorimetrical  estimations  have  to  be 
made  it  will  be  found  convenient  to  modify  the  above  method  in  the 
following  manner : — The  weighed  quantity  of  arsenious  acid  is  dissolved 
by  a  gentle  heat  in  10  or  15  c.c.  of  glycerine,  and  diluted  with  water  to 
I  litre.  The  weighed  sample  of  bleaching  powder  is  treated  with 
water  as  directed,  and  a  portion  of  the  turbid  solution  poured  into  a 
burette.  25  c.c.  of  the  standard  arsenious  acid  solution  are  delivered 
into  a  flask,  mixed  with  I  cubic  centimetre  of  indigo  solution,  and  the 
bleaching  powder  solution  added,  with  constant  shaking,  until  the  blue 
colour  is  discharged. 


Bleaching  Powder.  197 

the  turbid  solution  required  47*3  cubic  centimetres  of  the 
arsenious  acid  solution  to  complete  the  reaction.  Since 
i  cubic  centimetre  of  this  solution  is  equivalent  to  0*003546 
of  chlorine,  this  would  correspond  to  47-3  x  0-003546  = 
0^1677  gram  in  the  50  cubic  centimetres  of  solution.  But  the 
50  cubic  centimetres  contain  0-5495  gram  of  the  bleaching 


powder  ;  hence  the   substance   contains    ___      I0°  = 

Q'5495 
30-52  per  cent,  of  chlorine. 

The  amount  of  calcium  chloride  present  in  a  sample  of 
bleaching  powder  may  be  determined  by  first  estimating  the 
hypochlorite  in  the  manner  above  described,  and  then 
adding  to  a  second  portion  of  50  cubic  centimetres  a  slight 
excess  of  ammonia,  and  warming.  The  hypochlorite  is  thus 
converted  into  the  chloride,  with  the  formation  of  water  and 
nitrogen  : 

3Ca(C10)2  +  4NH3     =     3CaCl2  +  6H2O  4-  4N. 

The  chlorine  is  then  determined  in  the  solution,  after 
boiling,  and  cautiously  neutralising  with  nitric  acid,  by 
means  of  standard  silver  solution.  In  normal  bleaching 
powder  the  amount  of  available  chlorine  will  be  equivalent 
to  that  existing  as  calcium  chloride.  To  determine  the 
amount  of  chlorate  present,  a  third  portion  is  heated  with 
ammonia,  strongly  acidified  with  pure  sulphuric  acid,  and 
digested  with  metallic  zinc.  In  a  few  hours  the  nascent 
hydrogen  will  have  completely  reduced  the  chloric  acid  to 
the  state  of  hydrochloric  acid,  and  on  again  precipitating 
the  chlorine,  the  increased  amount  over  the  second  determi- 
nation shows  the  quantity  existing  as  chlorate. 

Another  method  of  estimating  the  total  chlorine  present 
in  bleaching  powder,  is  to  boil  the  turbid  solution  with  a 
solution  of  ferrous  sulphate  and  potash,  whereby  the  hypo- 
chlorous  and  chloric  acids  are  reduced  to  hydrochloric  acid  : 

HC10  +   HC103  +   8FeO     =     2HC1 


198  Quantitative  Chemical  Analysis. 

The  solution  is  filtered,  acidified  with  nitric  acid,  and  the 
chlorine  precipitated  with  silver  nitrate. 

In  some  parts  of  the  Continent,  particularly  in  France,  it 
is  customary  to  represent  the  amount  of  available  chlorine, 
not  in  percentages,  but  in  chlorimetrical  degrees,  represent- 
ing the  number  of  litres  of  chlorine  at  o°,  and  760  milli- 
metres which  i  kilo,  of  the  sample  should  yield.  Thus,  if  a 
sample  is  reported  to  be  of  100°,  it  means  that  i  kilo,  of  it 
would  yield  100  litres  of  chlorine,  measured  at  the  standard 
temperature  and  pressure.  Since  a  litre  of  chlorine  weighs 
3-177  grams,  this  sample  would  contain  in  a  kilo.  3177 
grams,  or  3177  per  cent.  Conversely  it  is  easy  to  see  that 
3177  per  cent,  would  be  equal  to  100°,  since  3177  per 

cent,  is  equal  to  3177  per  mille,  and  3I1_7    =    i0o°. 

Bunseifs  Method  (Modified.} — Withdraw  20  cubic  centi- 
metres of  the  turbid  solution,  made  in  the  manner  above 
described,  and  place  it  in  a  beaker,  add  about  15  cubic 
centimetres  of  potassium  iodide  solution,  acidify  with  hydro- 
chloric acid,  and  determine  the  iodine  liberated  according 
to  the  method  given  on  p.  157.  The  amount  of  available 
chlorine  in  the  sample  is  thus  measured  by  the  quantity  of 
iodine  which  it  can  set  free. 

XL     BLACK-ASH  ;   SODA- ASH  ;   VAT- WASTE. 

Black-ash  is  the  product  obtained  by  heating  the  mixture 
of  sodium  sulphate  (salt-cake),  calcium  carbonate,  and  small 
coal  or  slack  in  a  reverberatory  furnace,  in  the  manufacture 
of  soda  by  Leblanc's  process.  It  consists  essentially  of  a 
mixture  of  carbonate  and  caustic  soda  with  sulphide  and 
carbonate  of  calcium.  In  addition  it  contains  small  quantities 
of  sodium  sulphite,  thiosulphate  (hyposulphite),  sulphide,  and 
undecomposed  sulphate  and  chloride,  together  with  alumina, 
ferrous  sulphide,  sand,  and  unburnt  carbon;  to  the  last-named 
substance  is  mainly  due  the  characteristic  colour  of  the  product. 

On  lixiviating  the  fused  mass  with  tepid  water,  the  greater 


£  lack- Ask,  &c.  199 

portion  of  the  sodium  compounds  pass  into  solution,  and  on 
evaporating  the  clear  liquid  crude  soda-ash  is  obtained.  The 
insoluble  matter  remaining  in  the  lixiviating  tanks,  and  con- 
sisting mainly  of  calcium  sulphide  and  carbonate,  is  termed 
tank-  or  vat-waste. 

The  difference  in  composition  of  these  various  products  is 
well  seen  in  the  following  analyses  :— 

I.  BLACK-ASH—GERMAN  MAKE  (ANALYSED  BY  FRESENIUS). 
Sodium  carbonate  .  .  31  -982 
Sodium  hydrate  .  .  6-104 
Sodium  silicate .  .  .  1-019 
Soluble  in  water  Sodium  aluminate  .  .  i  -080 
Sodium  sulphide  .  .  0-133 
Sodium  sulphite  .  .  0-216 
Sodium  chloride  .  .  0*288 

40-822 

C  Calcium  sulphide  *     . 
Calcium  carbonate 
Lime 

I  Ferrous  sulphide 
Insoluble  in  water  •(  Silica 

I  Alumina   . 
Soda 
Carbon 
IjSand        ... 

59-026 

99-848 

II.  REFINED  SODA-ASH^-GLASGOW  (ANALYSED  BY  BROWN). 

Sodium  carbonate  .         .          .         .         .  .80-92 

Sodium  hydrate  .         .          .         .         .  .3-92 

Sodium  silicate   .  .         .         .         .         .  .1-32 

Sodium  aluminate  .         .         .         .         .  I  "Oi 

Sodium  sulphate  .         .         .         .         .  7 '43 

Sodium  sulphite  .          .         .         .         .  .I'll 

Sodium  thiosulphate  ......     trace 

Sodium  sulphide  .          .         .         .         .  .0*23 

Sodium  chloride  .         .         .         .          .  .3-14 

Insoluble  matter  .         .         .         .         .  .0-77 

99-85 

*  The  combination  of  lime  and  sulphur  in  the  portion  insoluble  in 
water  has  been  rearranged  in  accordance  with  the  views  now  generally 
held  as  to  the  manner  in  which  these  bodies  are  united  in  the  vat-waste. 
In  the  original  analyses  the  sulphur  (excluding  that  present  in  the  ferrous 
sulphide)  was  calculated  to  calcium  oxysulphide,  3CaS.CaO. 


2OO  Quantitative  Chemical  Analysis. 

The  following  scheme  gives  the  method  for  the  complete 
analysis  of  black-ash. 

About  30  grams  of  the  finely-powdered  ash  are  digested 
with  water  at  a  temperature  of  about  45°  in  a  flask  of  500 
c.c.  capacity.  The  solution  should  be  hastened  as  far  as 
possible  by  repeated  shaking ;  the  insoluble  matter  is  then 
allowed  to  subside,  and  in  a  few  hours  the  clear  supernatant 
liquid  is  poured  through  a  folded  filter  into  a  J-litre  flask. 
The  residue,  which  should  be  kept  as  far  as  practicable 
in  the  flask  in  which  it  was  originally  placed,  is  quickly 
washed  with  cold  water,  and  the  washings  added  to  the 
main  bulk  of  the  filtrate.  Discontinue  the  washing  when 
the  filtrate  commences  to  be  turbid,  and  fill  up  the  1-litre 
flask  to  the  containing-mark.  Close  and  shake  it.  If  the 
knowledge  of  the  nature  of  the  soluble  matter  is  not  im- 
mediately wanted,  it  is  better  to  proceed  at.  once  with  the 
examination  of  the  insoluble  portion,  since  this  is  apt  to 
suffer  alteration  on  standing. 

A.     Analysis  of  Insoluble  Matter. 

Wash  the  precipitate  from  the  filter  back  again  into  the 
flask  in  which  the  main  portion  of  the  residue  is  contained, 
and  without  delay  attach  the  flask  to  the  rest  of  the  apparatus 
represented  in  fig.  55,  the  several  parts  of  which  should  be 
already  weighed,  as  indicated  below,  and  put  together.  A  is 
the  flask  containing  the  insoluble  matter.  It  is  fitted  with  a 
caoutchouc  cork,  through  which  is  passed  a  100  c.c.  pipette 
filled  with  hydrochloric  acid  of  sp.  gr.  i  *i.  To  the  upper  end 
of  the  pipette  is  attached  a  piece  of  caoutchouc  tubing  which 
can  be  closed  by  a  screw  clamp.  The  other  end  of  the  tube 
is  connected  with  a  soda-lime  tube,  s  ;  A  is  connected  with 
the  flask  B,  of  300  c.c.  capacity,  and  containing  a  cold 
saturated  solution  of  copper  acetate  free  from  sulphuric  acid. 
Both  flasks  stand  on  an  iron  plate  which  can  be  heated  by  a 
lamp ;  B  is  connected  with  the  two-bulbed  U-tube  <:,  also 
containing  copper  acetate  solution,  standing  in  an  empty 


Black- Ash,  &c. 


201 


beaker,  and  fitted  with  bent  tubes  in  such  a  manner  that 
the  gas  traversing  the  apparatus  passes  twice  through  the 


FIG.  55. 


same  quantity  of  liquid  (fig.  56).  The  two  U-tubes  d  and 
e  are  filled  with  calcium  chloride  to  dry  the  gas  :  f  is  a 
potash-apparatus  (of  the  form  known  as  F 

Geissler's) ;  it  is  filled  to  the  extent  indi- 
cated in  the  figure  with  solution  of  potash 
of  sp.gr.  1-27  (containing  about  30  per 
cent,  of  potassium  hydrate),  and  is  ac- 
curately weighed  :  g  and  h  are  U-tubes 
filled  with  soda-lime  and  calcium  chloride  : 
g  only  is  weighed,  its  object  is  to  absorb 
the  last  traces  of  carbon  dioxide ;  h  is 
placed  merely  to  prevent  the  absorption 
of  atmospheric  carbon  dioxide  and  moisture  by  g :  the 
caoutchouc  tube  at  the  end  leads  to  the  filter-pump  or 
other  aspirating  arrangement.  As  soon  as  the  apparatus  is 
put  together,  fill  the  pipette  with  hydrochloric  acid,  fit  the 


2O2  Quantitative  Chemical  A  natysis. 

cork  into  A  and  open  the  clamp,  so  as  to  cause  the  acid  to 
enter  the  flask.  The  insoluble  matter  in  A  is  immediately 
decomposed,  tand  sulphuretted  hydrogen  and  carbon  dioxide 
are  simultaneously  evolved ;  the  former  is  absorbed  in  B 
and  c  ;  the  latter,  after  being  dried  by  passing  through  d  and 
^,  is  absorbed  by/  and  g.  When  the  evolution  of  the  gas 
slackens,  add  more  acid,  until  the  decomposition  is  complete. 
Heat  the  iron  plate  until  the  contents  of  A  and  B  are  in  gentle 
ebullition,  pour  hot  water  into  the  beaker,  open  the  clamp, 
and  aspirate  a  slow  current  of  air  (about  5  litres)  through  the 
apparatus. 

The  increase  in  the  weight  of/  and  g  gives  the  amount  of 
carbon  dioxide.  The  copper  sulphide  is  thrown  on  to  a  filter, 
and,  without  washing,  the  precipitate,  together  with  the  filter, 
is  transferred  to  a  flask  and  treated  with  hydrochloric  acid  and 
potassium  chlorate.  The  liquid  is  filtered  into  a  litre  flask, 
the  filter  washed,  and  the  filtrate  diluted  to  the  containing- 
mark.  Withdraw  two  portions  of  100  c.c.,  and  determine  in 
each  the  sulphuric  acid  by  means  of  barium  chloride.  233-2 
parts  of  barium  sulphate  are  equivalent  to  32  parts  of  sulphur. 

Pour  the  contents  of  the  flask  A  on  to  a  weighed  filter, 
receiving  the  filtrate  in  a  J-litre  flask,  wash  the  insoluble 
matter,  consisting  of  sand  and  carbon,  dry  at  100°  and 
weigh.  Ignite  the  weighed  mixture  to  burn  off  the  carbon, 
and  weigh  the  residual  sand.  The  difference  between  the 
two  weighings  gives  the  carbon. 

Make  up  the  filtrate  to  500  c.c.,  and  transfer  200  c.c.  to 
a  porcelain  basin,  add  a  small  quantity  of  nitric  acid,  and 
evaporate  to  dryness  on  the  water-bath.  Treat  the  separated 
silica  in  the  usual  manner  and  weigh  it.  Precipitate  the 
iron  and  alumina  from  the  filtrate  by  ammonia,  weigh 
them  together,  fuse  the  mixture  with  a  little  acid  potassium 
sulphate,  and  determine  the  iron  volumetrically  by  potas- 
sium permanganate.  The  alumina  is  found  by  difference. 
Determine  the  lime,  magnesia,  and  alkalies  by  the  method 
given  on  p.  184. 


Black- Ash,  &c.  203 

B.     Analysis  of  the  Soluble  Portion. 

1.  Withdraw  50   c.c.,  and  determine  the  total  alkali  pre- 
sent by  standard  acid,  litmus,  and    soda,  in    the  manner 
directed  on  p.  139.     Calculate  as  sodium  carbonate. 

2.  Transfer  100   c.c.  to  a  J-litre  flask,  mix  with  barium 
chloride  solution  so  long  as  a  precipitate  forms,  fill  up  the 
flask  to  the  containing-mark,  shake,  and  close  it.     Determine 
the  sodium  hydrate  in  an  aliquot  portion  of  the  clear  liquid 
by  standard  acid  and  litmus. 

3.  Transfer  50  c.c.  to  a  large  beaker,  dilute  with  200  c.c. 
of  water,  add   acetic  acid  until  the    liquid   is  very  nearly 
neutral,  and  determine  the  joint  amount  of  the  sulphide  and 
sulphite  of  sodium  by  means  of  starch  paste  and  standard 
iodine  solution. 

4.  Transfer    100   c.c.    to  a  i-litre   flask,'  and  add  zinc 
sulphate  solution  made  strongly  alkaline  by  potash,  until  a 
considerable  precipitate  is  formed.     This  contains  the  whole 
of  the  sulphur  present  as  sodium  sulphide ;  the  rest  of  the 
sulphur  remains  in  the  solution.     Dilute  to  the  containing- 
mark,  shake,  allow  to  settle,  and  determine  the  sulphur  still 
in  solution  in  an  aliquot  portion  of  the  clear  liquid  by  means 
of  starch  and  standard  iodine  solution,  after  acidifying  with 
acetic  acid  in  the  manner  previously  directed.     From  the 
amount  of  iodine  used,  the  sulphur  dioxide  is  readily  cal- 
culated: 2537  parts  of  iodine  are  equivalent  to  126*15  of 
sodium  sulphite.     The  difference  between  this  and  the  pre- 
vious determination  of  the  total  amount  of  iodine  required 
gives  the  quantity  of  sodium  sulphide  :  2537  parts  of  iodine 
correspond  to  78*15  of  sodium  monosulphide. 

5.  Evaporate   100  c.c.    to    dryness    in  a  thin   porcelain 
basin  with  a  small  quantity  of  pure  nitre,  and  gently  fuse 
the  residue.     The  sodium  sulphide  and  sulphite  are  thereby 
oxidised  to  sulphate.     Digest  the  mass  with  hot  water,  filter 
the  solution  into  a  250  c.c.  flask,  wash  the  insoluble  matter, 
and  fill  up  the  flask  to  the  mark,  and  shake.     Withdraw  100 


204  Quantitative  Chemical  Analysis. 

c.c.,  and  determine  the  sulphuric  acid,  after  acidulating  with 
hydrochloric  acid,  as  barium  sulphate.  Subtract  the  amount 
corresponding  to  the  sodium  sulphide  and  sulphite  ;  the 
remainder  is  calculated  to  sodium  sulphate.*  In  another  100 
c.c.  determine  the  chlorine  by  standard  silver. 

6.  Transfer  IOQC.C.  to  a  porcelain  dish,  acidify  with  hydro- 
chloric acid,  and  separate  the  silica  in  the  usual  way.  In  the 
filtrate  determine  the  alumina  by  precipitation  with  am- 
monia. 

Arrangement  of  the  Results. — A.  Insoluble  Portion. — Cal- 
culate the  iron  to  ferrous  sulphide,  and  the  remainder  of  the 
sulphur  to  calcium  sulphide,  CaS.  The  rest  of  the  calcium 
is  to  be  set  down  as  lime,  CaO.  The  silica,  alumina,  and  soda 
are  set  down  uncombined,  as  we  have  no  knowledge  of  their 
state  in  the  insoluble  portion.  The  sand  and  carbon  are,  of 
course,  directly  determined. 

B.  Soluble  Portion. — Combine  the  silica  and  alumina 
with  soda,  to  form  sodium  silicate  and  aluminate,  NazSiO3, 
and  Na2Al2O4  ;  calculate  the  amount  of  sodium  carbonate 
equivalent  to  these  compounds,  together  with  that  corre- 
sponding to  the  hydrate  and  sulphide,  and  subtract  the  joint 
amount  from  the  result  obtained  by  determining  the  total 
alkali  with  standard  acid  and  soda.  The  remainder  gives 
the  real  sodium  carbonate  present  in  the  solution. 

The  foregoing  scheme  of  analysis  is,  of  course,  equally 
applicable  to  the  analysis  of  soda-ash  and  vat-waste.  Vat- 

*  It  is  sometimes  requisite  for  the  purposes  of  the  manufacturer  merely 
to  determine  the  quantity  of  sodium  sulphide  in  the  liquor.  This  may 
be  readily  effected  by  the  following  process,  due  to  Lestelle.  The  solu- 
tion is  mixed  with  ammonia,  heated  to  boiling,  and  titrated  with  a  weak 
standard  solution  of  silver,  made  by  dissolving  4*3475  grams  of  pure 
silver  nitrate  in  water  in  a  litre  flask,  adding  excess  of  ammonia,  and 
diluting  to  1,000  c.c.  I  c.c.  is  equivalent  to  I  milligram  of  sodium  sul- 
phide. When  the  sulphur  is  nearly  all  precipitated,  the  liquid  is 
filtered,  and  the  addition  of  the  silver  solution  continued  until,  after  again 
filtering,  only  the  faintest  turbidity  is  produced.  The  method  is  expedi- 
tious, and  after  a  little  practice  gives  accurate  results. 


Copper  Ores.  205 

waste  is  treated  exactly  like  the  insoluble  portion  of  black- 
ash  ;  soda-ash  like  the  soluble  portion. 

XII.  ESTIMATION  OF  SULPHUR  IN  PYRITES,  BY  MEANS 

OF  COPPER  OXIDE. 

(Particularly  applicable  to  roasted  Pyrites.) 

A  rapid  and  sufficiently  accurate  method  for  technical 
purposes  consists  in  heating  from  5  to  10  grams  of  the  ore, 
according  to  its  richness  in  sulphur  (if  more  than  10  per 
cent,  of  sulphur  is  present,  5  grams,  if  less  than  10  per  cent, 
10  grams  are  taken),  intimately  mixed  with  5  grams  of  pure 
sodium  carbonate,  and  about  50  grams  of  dry  copper  oxide, 
in  a  porcelain  crucible,  to  a  low  red  heat,  for  about  a  quarter 
of  an  hour,  with  frequent  stirring.  The  sulphur  is  oxidised  to 
sulphuric  acid,  and  combines  with  the  alkali.  When  cold 
the  mass  is  treated  with  water,  and  the  amount  of  sodium 
carbonate  remaining  determined  by  titration  with  normal 
acid. 

XIII.  ASSAY  OF  COPPER  ORES  (MANSFELD  PROCESS). 

About  5  grms.  of  the  finely-powdered  ore  are  weighed  out 
into  a  flask,  and  mixed  with  40  c.c.  of  moderately-concen- 
trated hydrochloric  acid  (sp.  gr.  n6).  6  c.c.  of  dilute 
nitric  acid  (made  by  mixing  equal  bulks  of  water  and  pure 
acid  of  sp.  gr.  1-2)  are  added,  and  the  flask  is  gently  heated 
for  30  minutes  on  a  sand-bath,  after  which  it  is  boiled  for 
15  minutes.  The  whole  of  the  copper  is  now  in  solution  : 
the  extraction  is  complete,  even  in  the  case  of  very  rich 
ores,  provided  sufficient  attention  has  been  paid  to  the 
powdering.  The  solution  is  filtered  into  a  large  beaker,  into 
which  a  rod  of  zinc,  weighing  about  50  grms.,  and  surrounded 
with  a  piece  of  thick  platinum  foil,  has  been  previously 
placed.  It  is  necessary  that  the  zinc  employed  should  be  as 
free  as  possible  from  lead.  The  precipitation  of  the  metallic 
copper  commences  immediately,  and  is  generally  complete 
in  about  half  an  hour.  The  rod  of  zinc  is  withdrawn,  and 


2o6  Quantitative  Chemical  A  nalysis. 

the  precipitated  copper  repeatedly  washed  by  decantation. 
If  the  amount  of  the  copper  does  not  exceed  6  per  cent, 
(which  may  be  approximately  known  from  the  bulk  of  the 
reduced  metal),  it  is  dissolved  in  8  c.c.  of  the  dilute  nitric 
acid,  prepared  as  above.  The  beaker  is  gently  warmed, 
and  the  amount  of  copper  in  the  liquid  titrated  by  solution 
of  potassium  cyanide,  after  previous  addition  of  10  c.c.  of 
ammonia  solution,  prepared  by  diluting  i  vol.  of  ammonia- 
water  (sp.  gr.  o'93)  with  2  vols.  of  water.  When  the 
amount  of  copper  in  the  ore  exceeds  6  per  cent,  the  metal 
is  dissolved  in  16  c.c.  of  the  nitric  acid  solution,  and  the 
liquid  is  washed,  into  a  100  c.c.  flask,  diluted  to  the  con- 
taining-mark,  shaken,  50  c.c.  withdrawn,  mixed  with- 10  c.c. 
of  the  dilute  ammonia,  and  titrated  with  potassium  cyanide. 
The  experiment  may  be  repeated  with  the  second  portion  of 
50  c.c. 

When  a  solution  of  potassium  cyanide  is  mixed  with  an 
ammoniacal  solution  of  copper  sulphate  or  nitrate,  the  azure 
blue  colour  gradually  disappears  with  the  formation  of 
copper-ammonium-cyanide,  free  ammonium  cyanide,  ammo- 
nium formate,  and  urea.  The  reaction  is  only  constant  so 
long  as  the  amount  of  free  and  combined  ammonia  present 
is  invariable. 

The  strength  of  the  solution  of  the  potassium  cyanide  is 
thus  tested :— Exactly  5  grams  of  chemically  pure  copper,  pre- 
pared by  the  electrotype  process,  are  weighed  out  into  a 
litre  flask,  and  dissolved  at  a  gentle  heat  in  266-6  c.c.  of  the 
dilute  nitric  acid  (made  in  the  manner  above  described). 
On  cooling,  the  solution  is  diluted  to  the  containing-mark. 
30  c.c.  of  this  solution,  containing  0-15  grm.  of  metallic 
copper,  are  placed  in  a  beaker  and  mixed  with  10  c.c.  of 
the  dilute  ammonia  liquid,  and  the  solution  of  potassium 
cyanide  is  added  from  a  burette,  with  constant  stirring,  until 
the  blue  colour  of  the  liquid  just  disappears.  The  strength 
of  the  cyanide  should  be  so  arranged  that  i  c.c.  of  the  solu- 
tion is  equivalent  to  5  milligrams  of  copper.  The  titration 


Copper  Ores.  207 

of  the  solution  of  the  sample  of  ore  is  made  in  exactly  the 
same  manner.  If  exactly  5  grams  have  been  taken,  and  the 
cyanide  is  of  the  above  strength,  each  cubic  centimetre  of 
the  solution  required  for  decolourisation  is  equivalent  to  o'i 
per  cent,  of  copper.  The  number  of  cubic  centimetres 
needed,  divided  by  10,  gives  the  percentage  of  copper  at 
once. 

This  method  is  very  expeditious,  and  if  due  care  be  exer- 
cised, it  is  very  accurate.  It  must  be  borne  in  mind  that  it 
is  strictly  comparative,  and  the  titrations  must  be  made 
therefore  in  a  uniform  manner.  The  presence  of  a 'very- 
small  quantity  of  lead  exercises  no  influence  on  the  results," 
but  the  action  of  zinc  is  more  injurious.  Care  must  be  taken 
therefore  to  wash  the  precipitated  copper  thoroughly  before 
dissolving  it  in  the  dilute  nitric  acid.  The  solutions  must 
be  quite  cold  before  titration,  since  less  potassium  cyanide 
is  needed  to  decolourise  a  solution  when  warm  than  when 
cold.  Thus  while  30  c.c.  of  copper  solution,  containing  0*15 
grm.  copper,  and  10  c.c.  normal  ammonia  solution,  required 
at  the  ordinary  temperature  exactly  30  c.c.  of  potassium 
cyanide  solution,  the  same  quantities  at  about  45°  required 
only  28-9  c.c.  (STEINBECK). 

The  solution  of  potassium  cyanide  requires  to  be  titrated 
from  time  to  time,  since  its  strength  is  not  invariable. 

XIV.     ASSAY  OF  COPPER  ORES  (LUCKOW'S  PROCESS).* 

This  process,  which  is  now  largely  used  in  many  German 
establishments,  depends  upon  the  fact  that  copper  is  pre- 
cipitated in  the  metallic  state  from  acid  solutions  by  a  weak 
galvanic  current.  The  operations  required  by  this  method 
may  be  best  described  under  the  following  heads  :  i.  Roast- 
ing the  ore  ;  2.  Solution  of  the  roasted  ore  ;  3.  Precipitation 
of  the  copper ;  4.  Weighing  the  copper. 

i.  Roasting  the  Ore. — This  operation  is  only  necessary 
when  the  ores  are  bituminous.  Weigh  out  about  2  grams  of 

*  Zeitsch.  fur  Anal.  Chemie,  Fresenius,  1869,  p.  i. 


208  Quantitative  Chemical  Analysis. 

the  finely-powdered  sample  into  a  thin  porcelain  crucible, 
and  heat  it  over  a  Bunsen  flame  for  10  minutes,  occasionally 
stirring  it  with  a  thick  platinum  wire,  so  as  to  expose  fresh 
surfaces  to  the  oxidising  action  of  the  air.  The  bituminous 
matter  and  the  greater  portion  of  the  sulphur  will  be  expelled 
at  the  expiration  of  this  time. 

2.  Solution  of  the  Ore. — The  roasted  powder  is  transferred 
to  a  small   flat-bottomed   beaker,  about  5   centimetres  in 
height  and  3  centimetres  wide,  and  treated  with  6  c.c.  of 
nitric  acid  of  sp.  gr.  1*2,  4  c.c.  of  a  dilute  sulphuric  acid 
(prepared  by  mixing  equal  volumes  of  the  strong  acid  and 
water),  and  25  drops  of  hydrochloric  acid.     The  addition  of 
the  sulphuric  acid  increases  the  oxidising  action  of  the  nitric 
acid,  and  converts  any  lime  which  may  be  present  into  the 
difficultly-soluble  calcium  sulphate.    The  liquid  is  evaporated 
to   complete  dryness   on  a   sand-bath,   the    beaker  being 
meanwhile  covered  with  a  funnel,  the  stem  of  which  has  been 
cut  off.      This  operation    requires    about  an    hour.      The 
addition  of  the  hydrochloric  acid  facilitates  the  evaporation 
and  decreases  the  tendency  of  the  liquid   to  spirt.     When 
dry,  break  up  the  mass  with  a  glass  rod. 

3.  Precipitation    of  the  Copper. — Wash  the  cover  inside 
and  out  with  dilute  nitric  acid  (i  vol.  of  acid  of  sp.  gr.  1-2 
diluted  with  6  vols.  of  water),  and  also  the  sides  of  the 
beaker,  until  it  is  about  half-filled  with  the  dilute  acid.     Add 
a  few  drops  of  a  strong  solution  of  tartaric  acid  (which  is  best 
preserved  in  a  beaker  simply  covered  with  a  piece  of  paper) 
to  the  liquid,  and  place  the  spiral,  represented  in  fig.  56^7, 
within    the  beaker.       This   spiral    consists  of  a  piece  of 
platinum  wire  about  18  centimetres  long,  and  i  millimetre 
thick  ;   two-thirds  of  it  are  so  bent  in  circles  that  the  straight 
portion  of  the  wire  projects  as  if  it  were  the  axis  of  the  spiral. 
The  outer  convolution  is  so  large  that  it  just  touches  the 
sides  of  the  beaker  :  the  vertical  portion  of  the  wire  is  there- 
fore  exactly  in  its  centre.      If  the  evaporation  has  been 
carefully  attended  to,  the  acid  liquid  remains  quite  clear : 


Copper  Ores. 


209 


should  it  be  turbid,  add  i  c.c.  of  a  concentrated  solution  of 
barium  nitrate,  and  agitate  the  mixture  by  the  aid  of  the 
platinum  spiral.  A  piece  of  stout  platinum  foil,  to  which  a 
thick  platinum  wire  has  previously  been  attached,  is  then  bent 
into  a  cylinder  of  3  centimetres  long  and  18  millimetres  in 
diameter  (fig.  56  &);  this  is  accurately  weighed,  and  supported 
in  the  beaker  about  i  millimetre  from  the  spirals  :  the  ver- 
tical portion  of  the  spiral  becomes  therefore  the  axis  of  this 


FIG.  56  a. 


FIG.  560. 


FIG.  56  c. 


cylinder.  The  wire  supporting  the  foil  is  fixed  by  means  of  a 
screw,  a,  to  the  arm  a  b  of  the  stand  (fig.  56  c\  the  other  screw, 
£,  holds  the  wire  leading  from  the  zinc  pole  of  a  constant 
battery.  A  small  screw  clamp,  c,  is  fastened  to  the  end  of 
the  platinum  spiral,  and  connects  the  arrangement  with  the 
other  pole  of  the  battery.  Immediately  the  circuit  is  closed, 
copper  commences  to  be  deposited  on  the  platinum  foil,  and 
bubbles  of  oxygen  are  given  off  from  the  spiral :  in  about  8 
hours  the  whole  of  the  metal  will  certainly  be  precipitated, 

p 


2io  Quantitative  Chemical  Analysis. 

even  from  rich  ores  and  by  a  weak  current.  A  stream  of 
water  is  run  into  the  beaker  so  as  to  displace  the  acid  liquid, 
which  is  allowed  to  flow  over  the  sides.  The  platinum 
cylinder  is  then  withdrawn  and  disconnected  from  the  stand, 
washed  with  hot  water,  and  then  with  a  few  drops  of  alcohol. 
It  is  heated  in  the  steam-bath,  and  weighed  when  cold.  Its 
increase  in  weight  gives  the  quantity  of  metallic  copper 
present  in  the  sample.  As  the  process  of  precipitation 
needs  no  superintendence,  it  may  be  allowed  to  go  on 
during  the  night :  if  the  operation  be  commenced  in  the 
evening,  the  reduced  copper  will  be  ready  for  washing 
and  weighing  in  the  morning.  The  copper  should  show  its 
characteristic  red  colour,  and  be  free  from  any  saline  deposit : 
the  absence  of  this  deposit  is  evidence  of  the  perfect  re- 
moval of  copper  from  the  liquid. 

XV.    ASSAY  OF  COPPER  ORES  BY  PRECIPITATING  THE 

METAL  BY  MEANS  OF  ZINC. 

(See  p.  105.) 

About  5  grams  of  the  finely-powdered  ore  are  gently 
heated  with  strong  aqua  regia  in  a  deep  porcelain  crucible, 
covered  with  a  watch-glass.  The  glass  is  rinsed  with  water, 
and  the  liquid  is  evaporated  to  complete  dryness.  The 
dried  residue  is  heated,  to  expel  the  unoxidised  sulphur  ;  if 
the  ore  contains  much  pyrites,  it  will  be  necessary  to  treat  it 
again  with  strong  nitric  acid  (B.P.  86°},and  evaporate  a  second 
time  to  dryness,  and  roast. 

Treat  the  dried  mass  with  hot  water  to  extract  the  copper 
(now  present  as  sulphate),  filter  into  a  weighed  platinum 
dish,  wash  the  insoluble  residue,  adding  the  washings  to  the 
solution  already  in  the  dish,  and  precipitate  the  copper  by 
means  of  zinc,  in  the  manner  directed  on  p.  105.  To  prove 
the  accuracy  of  this  method,  i  gram  of  pure  metallic  copper 
was  mixed  with  0*5  gram  of  the  following  substances,  either 
in  the  metallic  state,  or  as  salts,  viz.  : — Gold,  silver,  platinum, 
tin,  lead,  iron,  zinc,  nickel,. 'cobalt,  bismuth,  arsenic,  ura- 


Copper  Pyrites.  2 1 1 

mum,  mercury,  molybdenum,  antimony,  sulphur,  silica,  and 
calcium  phosphate.  The  mixture  was  treated  in  the  manner 
described,  and  the  copper  precipitated  by  zinc  ;  the  amount 
of  reduced  copper  was  0-996  gram.  In  more  than  twenty 
determinations  of  copper  in  various  combinations,  the 
average  amount  of  the  metal  obtained  by  this  method  was 
997  per  cent,  of  the  actual  quantity  present.  (MoHR.) 

XVI.     COPPER  PYRITES. 

This  mineral  constitutes  the  most  abundant  ore  of  copper  : 
it  consists  essentially  of  sulphur,  iron,  and  copper,  and 
when  pure  contains  34*8  per  cent,  of  copper.  It  is  almost 
invariably  mixed,  however,  with  more  or  less  antimony, 
arsenic,  bismuth,  lead,  manganese,  zinc,  nickel,  and  cobalt 
in  addition  to  considerable  quantities  of  silicious  substances 
and  gangue.  When  a  complete  quantitative  analysis  of  the 
ore  is  to  be  made,  it  must  always  be  preceded  by  a  careful 
qualitative  examination. 

Pulverise  about  20  grams  of  the  mineral  in  an  agate 
mortar,  and  dry  at  100°. 

Determination  of  the  Sulphur. — About  i  gram  of  the  ore 
is  weighed  out  into  a  porcelain  dish,  a  few  crystals  of  potas- 
sium chlorate  are  added,  together  with  about  50  cubic  centi- 
metres of  pure  nitric  acid  (sp.  gr.  1*35),  and  the  mixture  is 
heated  on  the  water-bath.  To  prevent  loss  by  spirting,  the 
dish  should  be  covered  with  a  large  watch-glass  or  funnel. 
Fig.  57.  As  the  evolution  of  chlorine  diminishes,  add  oc- 
casionally a  crystal  of  potassium  chlorate.  In  about  an  hour 
the  whole  of  the  sulphur  will  be  oxidised ;  the  cover  is  rinsed 
with  hot  water,  and  the  liquid  -in  the  dish  concentrated  to  a 
small  bulk,  a  small  quantity  of  strong  hydrochloric  acid  is 
added,  and  the  solution  is  evaporated  to  perfect  dryness,  in 
order  to  render  the  silica  insoluble.  Moisten  the  dried  re- 
sidue with  strong  hydrochloric  acid,  add  hot  water,  filter,  and 
•wash  the  insoluble  portion  by  decantation.  This  consists 

P  2 


?  1  2 


Quantitative  Chemical  A  nalysis. 


FIG 


mainly  of  silica  and  imdecomposed  gangue  ;  it  may  contain, 
however,  a  small  quantity  of  lead  sulphate,  left  undissolved 
by  the  hydrochloric  acid.  The  last  traces  of  the  sulphate 
may  be  removed  by  heating  the  mass 
with  a  solution  of  ammonium  acetate 
(made  by  mixing  solutions  of  am- 
monia and  acetic  acid),  and  adding 
the  liquid  to  the  main  quantity  of  the 
filtrate.  Add  to  the  solution  a  few 
crystals  of  tartaric  acid,  to  prevent  the 
precipitation  of  traces  of  iron,  heat 
the  liquid  to  boiling,  and  add  excess 
of  barium  chloride.  Allow  the  liquid 
to  stand  ;  filter,  wash  the  precipitate 
by  decantation  with  hot  water,  and 
digest  it  with  a  solution  of  ammonium 
acetate,  which  removes  any  traces  of 
barium  nitrate,  which  the  precipitate 
is  very  apt  to  retain.  The  barium 
sulphate  is  then  washed  on  to  the  filter,  dried,  and  weighed. 

Determination  of  the  Copper,  Iron,  &>c.  —  2  or  3  grams  of 
the  powdered  ore  are  oxidised  with  fuming  nitric  acid  in  a 
dish,  a  few  cubic  centimetres  of  strong  sulphuric  acid  are 
added,  and  the  liquid  evaporated  to  dryness.  The  residue  is 
dissolved  in  hydrochloric  acid,  water  added,  the  liquid 
allowed  to  stand,  and  then  filtered  into  a  flask.  The  insolu- 
ble portion  consists  mainly  of  silica  and  gangue,  but  it  may 
still  retain  a  small  quantity  of  lead  as  sulphate  ;  this  is  to  be  re- 
moved by  repeatedly  boiling  the  residue  in  the  dish  with  dilute 
hydrochloric  acid,  and  passing  the  liquid  through  the  filter. 

The  residue  is  then  transferred  to  the  filter,  washed  with 
hot  water,  dried,  and  weighed.  The  filtrate  is  warmed  to 
about  70°,  and  a  current  of  sulphuretted  hydrogen  passed 
through  it.  The  copper,  lead,  bismuth,  tin,  arsenic,  and 
antimony  are  precipitated.  Allow  the  liquor  to  stand,  pour 


Copper  Pyrites.  213 

the  supernatant  liquid  through  a  filter,  and  wash  by  de- 
cantation  with  water  containing  a  little  sulphuretted  hydro- 
gen. The  small  portion  of  the  sulphide  adhering  to  the 
filter  is  washed  back  into  the  flask,  and  the  precipitate  is 
gently  heated  with  a  moderately  concentrated  solution  of 
potassium  sulphide  ;  after  a  short  digestion  (thirty  or  forty 
minutes)  add  water  (otherwise  a  little  copper  may  remain 
in  solution),  and  again  filter.  The  sulphides  of  arsenic  and 
antimony,  mixed  with  sulphur,  obtained  by  adding  hydro- 
chloric acid  to  the  filtrate,  are  filtered  off,  and  oxidised  by  red 
fuming  nitric  acid  (B.P.  86°),  the  solution  concentrated,  an 
excess  of  sodium  carbonate  added,  and  the  whole  evapo- 
rated to  complete  dryness,  and  fused  in  a  silver  dish.  The 
fused  mass  is  further  treated  as  in  No.  XV.  Part  II. 

The  sulphide  of  copper  containing  the  small  quantities  of 
lead  and  bismuth,  is  dried  and  transferred  to  a  small  porce- 
lain basin,  and  dissolved  in  nitric  acid.  The  solution  is 
evaporated  to  a  small  bulk,  chloride  of  ammonium  added  to 
dissolve  the  bismuth,  and  then  dilute  sulphuric  acid.  Allow 
the  precipitate  to  settle  completely,  pour  off  the  clear  liquid 
through  a  filter,  quickly  wash  twice  or  three  times  with  a 
little  water  containing  a  drop  or  two  of  sulphuric  acid,  rinse 
the  lead. sulphate  on  to  the  filter  by  means  of  alcohol,  and 
wash  the  filter  paper  thoroughly  with  alcohol.  Do  not  mix 
the  alcoholic  washings  with  the  main  quantity  of  the  filtrate. 
The  bismuth  is  best  precipitated  as  carbonate.  Nearly 
neutralise  the  filtrate  containing  the  copper,  and  add  excess 
of  ammonium  carbonate,  gently  warm,  filter,  dissolve  the 
precipitate,  which  consists  partly  of  bismuth  carbonate, 
partly  of  basic  bismuth  sulphate  mixed  with  copper,  in  a 
few  drops  of  nitric  acid,  and  again  add  ammonium  carbo- 
nate, which  reprecipitates  the  bismuth  as  pure  carbonate  free 
from  copper,  and  heat  gently  for  some  time  ;  wash,  dry,  and 
ignite  the  precipitate,  and  weigh  it  as  bismuth  trioxide, 
taking  care  to  detach  the  dried  carbonate  as  completely  as 
possible  from  the  paper  before  incineration.  Boil  the  solu- 


214  Quantitative  Chemical  A  nalysis. 

tion  containing  the  copper,  add  caustic  soda,  and  continue 
the  boiling  until  the  solution  is  free  from  ammonia,  filter, 
wash,  dry,  and  ignite. 

The  filtrate  from  the  original  precipitate  by  sulphuretted 
hydrogen  contains  the  iron,  zinc,  nickel,  cobalt,  and  man- 
ganese. It  is  concentrated  slightly,  mixed  with  nitric  acid, 
and  boiled  until  the  iron  is  peroxidised,  allowed  to  cool, 
mixed  with  a  large  quantity  of  strong  solution  of  am- 
monium chloride,  and  then,  drop  by  drop,  with  ammonium 
carbonate,  until  the  fluid  is  just  turbid  (it  must  not  show  the 
least  trace  of  distinct  precipitate).  Heat  to  boiling,  and 
maintain  the  fluid  in  ebullition  until  the  carbonic  acid  has 
been  expelled.  Allow  the  precipitate  to  settle,  add  one 
drop  of  ammonia  to  the  clear  liquid,  and,  if  no  precipitation 
ensues,  a  few  cubic  centimetres  of  ammonia  :  filter,  and  wash 
the  precipitate  with  water  containing  a  little  ammonium 
chloride.  Dry,  ignite,  and  weigh  the  ferric  oxide.  The 
filtrate  contains  the  manganese,  nickel,  cobalt,  and  zinc. 
Concentrate  to  a  small  bulk,  and  then  add  a  strong  solution 
of  sodium  acetate  and  acetic  acid  until  the  fluid  is  distinctly 
acid,  heat  to  boiling,  and  whilst  hot  pass  a  rapid  current  of 
sulphuretted  hydrogen  into  the  liquid.  The  zinc,  nickel,  and 
cobalt  are  precipitated ;  the  manganese  remains  in  solution. 
The  precipitate  is  thrown  upon  a  filter  and  washed  with 
water  containing  sulphuretted  hydrogen. 

The  manganese  in  the  filtrate  is  determined  by  boiling 
the  liquid  to  expel  the  sulphuretted  hydrogen,  adding  caustic 
soda  until  it  is  nearly  neutral,  and  then  a  few  drops  of 
bromine,  and  wanning  until  the  manganese  separates  out  as 
binoxide.  This  is  filtered,  well  washed,  and  converted  into 
protosesquioxide  (Mn3O4)  by  ignition.  The  filtrate  will 
contain  any  lime  and  magnesia  derived  from  the  gangue ; 
these  earths  may  be  separated  in  the  usual  manner  by  am- 
monium oxalate  and  sodium  phosphate. 

The  mixed  sulphides  of  zinc,  nickel,  and  cobalt  on  the 
filter  are  dried,  transferred  to  a  small  beaker,  the  filter  burned, 


Iron  Pyrites,  &c.  215 

and  the  ash  added,  and  the  whole  dissolved  in  dilute  aqua 
regia  ;  solution  of  caustic  potash  is  added  until  the  solution 
is  slightly  alkaline,  then  acetic  acid  until  it  is  distinctly  acid, 
and  lastly  a  strong  solution  of  potassium  nitrite.  Allow  the 
solution  to  stand  for  at  least  24  hours,  take  out  a  small  por- 
tion of  the  clear  liquid,  and  add  to  it  a  few  more  drops  of 
potassium  nitrite  ;  if  after  the  lapse  of  a  couple  of  hours 
no  further  precipitation  ensues,  the  separation  of  the  cobalt 
is  complete.  Pass  both  portions  of  the  solution  through 
a  small  filter,  wash  and  dry  the  yellow  crystalline  potassium- 
cobalt  nitrite,  transfer  it  to  a  small  crucible,  incinerate  the 
filter,  add  the  ash,  moisten  the  whole  with  strong  sulphuric 
acid,  expel  the  excess  by  heat,  and  weigh  the  residue  ;  it  has 
the  composition  2CoSO4  +  3K2SO4,  and  contains  18*0  per 
cent,  of  cobalt  oxide. 

The  filtrate  containing  the  zinc  and  nickel  is  boiled,  car- 
bonate of  soda  added,  and  the  metals  separated  as  in 
No.  XIV.  Part  II.* 

XVII.     IRON  PYRITES 
may  be  analysed  by  the  methods  adopted  for  copper  pyrites. 

XVIII.      c  KUPFERNICKELSTEIN  ' 

is  a  mixture  of  sulphides  of  copper  and  nickel,  containing 
arsenic,  iron,  cobalt,  lead,  &c.  It  is  obtained  as  an  inter- 
mediate product  in  the  preparation  of  copper-nickel  and 
German  silver.  It  may  be  analysed  by  the  processes  described 
under  copper  pyrites. 

XIX.     IRON-ORES. 

The  ores  of  iron  more  commonly  used  for  the  extraction 
of  the  metal  are  the  magnetic  oxide,  red  and  brown  haematite, 
specular  ore,  spathic  ore,  and  clay  iron-stone.  The  following 
analyses  show  the  characteristic  features  of  these  varieties. 

*  If  the  copper  alone  is  to  be  determined  in  the  ore,  the  above  pro- 
cess is  recommended  whenever  an  accurate  estimation  is  required. 


216 


Quantitative  Chemical  Analysis. 


i 

2 

3 

4 

5 

« 

Ferric  oxide 

70-23 

94-23 

90-05 

2'7S 

2-72 

0-40 

Ferrous  oxide     . 

29-65 



48-I2 

40-77 

45-86 

Manganous  oxide 

0-23 

0-88 

0-83 

o-q6 

Alumina    . 

0-63 

0-14 

I  "63 

5-86 

Lime 

0-05 

0-06 

i-75 

0-90  !    1-37 

Magnesia  . 

trace 

O'2O 

2-29 

0-72      1-85 

Silica 

4-90 

0-92 

1-62 

lo-io      10-88 

Carbonic  acid     . 



39-92 

26-41 

31  -02 

Phosphoric  acid. 

trace 

0-09 

°'54 

O'2I 

Sulphuric  acid 
Iron  Pyrites 

0-09 
0-03 

>  traces 

0-22 

— 

trace 
o-io 

Water        .         ] 

0-56 

9-22 

°"45 

I'OO 

i  -08 

Organic  matter 

— 

— 

0-39 

17-38 

0-90 

99-88 

100-72 

100-76 

100-51 

100-00 

100-29 

1.  Magnetic  ore.     Dannemora. 

2.  Red  Haematite.     Ulverstone. 

3.  Brown  Haematite.     Dean  Forest. 


4.  Spathic  ore.      Westphalia. 

5.  Blackband  ore.     Scotland. 

6.  Clay  iron-stone.      Dudley. 


General  Method  for  the  Complete  Analysis  of  Iron  Ores. — 
The  ore  is  carefully  sampled,  and  an  average  portion  of  it  is 
reduced  to  a  moderately  fine  powder,  dried  under  the  desic- 
cator or  at  1 00°,  according  to  circumstances,  and  kept  in  a 
well- corked  tube  or  bottle. 

Determination  of  the  Moisture. — In  ores  containing  car- 
bonic acid,  and  organic  matter,  the  water  can  only  be 
estimated  by  direct  weighing.  2  or  3  grams  of  the  powdered 
ore  are  introduced  into  the  bulb-tube  (fig.  53),  and  ignited 
in  a  slow  current  of  air  dried  by  the  chloride  of  calcium  tube: 
the  water  is  condensed  in  the  weighed  tube  containing 
calcium  chloride.  The  rate  at  which  the  air  passes  through 
the  apparatus  is  seen  in  the  small  flask  containing  sulphuric 
acid.  After  an  hour's  gentle  ignition  the  tube  is  re-weighed  ; 
its  increase  in  weight  shows  the  amount  of  moisture  present 
in  the  sample. 

Determination  of  the  Carbonic  Add. — This  is  best  effected 
by  means  of  the  apparatus  represented  in  fig.  31,  p.  86. 


Iron   Ores.  217 

Weigh  out  i  or  2  grams  of  the  ore  into  the  flask  A,  and  pro- 
ceed as  directed  (see  No.  V.  Part  II.). 

Determination  of  the  Silica,  Iron,  Manganese,  Sulphur, 
Phosphorus,  Alumina,  Lime,  Magnesia,  &>c. — Introduce 
about  8  or  10  grams  of  the  finely-powdered  ore  into  a  porcelain 
basin,  and  gently  heat  with  concentrated  hydrochloric  acid, 
mixed  with  a  little  nitric  acid,  until  the  mineral  is  completely 
decomposed.  Some  varieties  of  ore — for  example,  blackband 
iron-stone — contain  such  an  amount  of  organic  matter  that  it 
is  difficult  to  determine  when  they  are  completely  dissolved. 
As  a  rule  not  more  than  30  or  40  minutes  will  be  required  for 
complete  decomposition.  Some  specimens  of  magnetic  iron- 
stone and  micaceous  haematites  dissolve  with  great  slowness 
even  in  concentrated  hydrochloric  acid.  In  such  cases  it  is 
better  to  heat  the  weighed  portion  of  the  finely-divided  ore  in 
a  current  of  hydrogen  or  coal  gas  until  water  ceases  to  be 
evolved  :  the  reduced  iron  will  then  readily  dissolve  in  the 
acid.  Fusion  with  acid  sulphate  of  potassium  also  effects  the 
decomposition  of  such  ores.  The  solution  is  evaporated  to 
dryness,  and  the  residue  drenched  with  strong  hydrochloric 
acid,  the  liquid  warmed,  diluted  with  hot  water,  allowed  to 
settle,  and  filtered  into  a  -J-litre  flask.  The  residue  in  the  dish 
is  again  heated  with  a  small  quantity  of  hydrochloric  acid, 
diluted  with  water,  allowed  to  settle,  and  the  clear  liquid 
poured  through  the  filter.  This  process  is  repeated  until  the 
silicious  residue  appears  quite  white  and  free  from  iron.  It  is 
then  thrown  on  to  the  filter  and  washed  thoroughly  with  hot 
water,  dried,  ignited,  and  weighed.  It  consists  of  gangue  and 
separated  silica.  The  amount  of  silica  may  be  determined  by 
boiling  the  weighed  residue  with  a  solution  of  sodium  carbo- 
nate in  a  platinum  dish,  filtering  and  determining  the  weight 
of  the  gangue  remaining  :  the  difference  gives  the  quantity  of 
silica.  It  is  sometimes  required  to  determine  the  nature  of  the 
gangue  :  this  is  accomplished  by  the  methods  given  in  Nos. 
XI.  &  XII.  Part  II.  Titanic  acid  is  not  an  unfrequent 


2 1 8  Quantitative  Chemical  A  nalysis. 

constituent  of  iron-ores  :  when  present  it  will  be  found  partly 
in  the  silica  separated  by  evaporation  to  dryness,  partly  in  the 
hydrochloric  acid  solution.  In  order  to  determine  its  amount 
a  large  quantity  of  the  ore  is  decomposed  by  hydrochloric 
acid,  in  the  manner  above  described,  evaporated  to  dryness, 
drenched  with  hydrochloric  acid,  and  the  residue  filtered  off 
and  washed.  The  silicious  matter  is  transferred  to  a  plati- 
num dish  and  repeatedly  treated,  in  a  *  draught-place '  or  in  the 
open  air,  with  hydrofluoric  acid,  and  a  little  sulphuric  acid;* 
the  residue  is  fused  with  potassium-hydrogen  sulphate,  dis- 
solved in  a  little  cold  water,  and  filtered,  if  necessary.  Am- 
monia is  added  in  slight  excess,  the  solution  boiled  for  some 
time,  filtered,  and  the  precipitated  titanic  acid  washed,  dried, 
ignited,  and  weighed.  To  the  hydrochloric  acid  solution  a 
few  cubic  centimetres  of  nitric  acid  are  added,  and  the  liquid 
is  boiled,  ammonia  added,  and  the  liquid  again  boiled.  The 
precipitated  oxide  of  iron  and  alumina  carries  down  the  rest 
of  the  titanic  acid  :  these  are  filtered  off,  washed,  dried,  and 
transferred  to  a  platinum  dish,  and  fused  with  potassium-hy- 
drogen sulphate.  The  mass  is  dissolved  in  a  large  quantity 
of  cold  water,  neutralised  with  sodium  carbonate,  and  solution 
of  sodium  thiosulphate  added  in  slight  excess.  When  the 
solution  becomes  nearly  colourless,  boil  until  all  sulphur' 
dioxide  is  expelled,  filter,  wash  the  precipitate  with  hot 
water,  dry,  and  ignite  it  gently  in  a  porcelain  crucible.  The 
residue  contains  all  the  titanic  acid  mixed  with  alumina.  It 
is  treated  with  strong  sulphuric  acid,  filtered  if  necessary, 
diluted  and  boiled  for  some  time,  and  the  separated  titanic 
acid  filtered  off  and  weighed. 

The  filtrate  from  the  silica,  &c,,  separated  from  the  10 
grams  of  iron,  is  diluted  to  500  cubic  centimetres  and  well 
mixed  by  shaking. 

Determination  of  the  Sulphur. — Take  out  100  cubic  centi- 

*  If  the  sulphuric  acid  be  omitted,  a  portion  of  the  titanium  will  be 
lost  by  volatilisation  as  fluoride. 


Iron  Ores.  219 

metres  of  the  solution  and  evaporate  nearly  to  dryness  to 
expel  the  greater  portion  of  the  free  acid,  dilute  with  about 
200  cubic  centimetres  of  water,  boil,  and  add  one  or  two 
drops  of  barium  chloride  solution.  After  standing  about 
24  hours  the  barium  sulphate  is  filtered  off  and  weighed. 

Determination  of  the  Phosphoric  and  Arsenic  Acids. — To 
TOO  cubic  centimetres  of  the  solution,  add  a  few  cubic  centi- 
metres of  a  clear  solution  of  molybdate  of  ammonium  in 
nitric  acid.  [This  solution  is  prepared  by  dissolving  10 
grams  of  powdered  ammonium  molybdate  in  40  c.c.  of  dilute 
ammonia  (sp.  gr.  0*96),  and  mixing  the  solution  with  i6oc.c. 
of  dilute  nitric  acid  (120  cubic  centimetres  of  strong  acid  to 
40  cubic  centimetres  of  water).  The  mixture  is  heated  to 
about  40°  for  some  hours,  and  the  clear  liquid  drawn  off.] 
The  mixture  of  iron  salt  and  molybdate  is  kept  in  a  warm 
place  (not  above  40°)  for  24  hours,  filtered,  and  washed,  the 
precipitate  treated  with  ammonia  on  the  filter,  and  magnesia- 
mixture  added  to  precipitate  the  dissolved  phosphoric  acid. 
After  standing  a  few  hours  the  magnesium  ammonium  phos- 
phate is  filtered,  and  treated  in  the  usual  manner.  Many  iron- 
ores  contain  notable  quantities  of  arsenic,  which  is  precipi- 
tated from  the  solution  on  adding  molybdic  acid.  If  quali- 
tative analysis  has  shown  the  presence  of  arsenic,  it  must  be 
removed  before  precipitating  the  phosphoric  acid  by  transmit- 
ting a  current  of  sulphuretted  hydrogen  through  the  100  cubic 
centimetres  of  iron  solution,  filtering,  heating  the  filtrate,  if 
turbid,  with  a  little  nitric  acid,  and  then  adding  the  molybdic 
acid  solution.  The  arsenic  in  the  precipitate  does  not 
represent  the  entire  amount  in  the  ore,  since  a  consider- 
able portion  is  lost  on  the  evaporation  of  the  solution  to 
dryness  in  order  to  render  the  silica  insoluble.  If  it  be 
desired  to  determine  the  amount  of  arsenic  actually  present, 
a  larger  quantity  of  the  ore  must  be  heated  with  aqua  regia, 
filtered,  and  treated  with  sulphuretted  hydrogen,  and  the 
arsenic  determined  in  the  precipitate  (see  No,  XVIII, 


22O  Quantitative  Chemical  Analysis. 

Part  II.)     Any  black  residue  left  after  oxidation  may  con- 
sist of  copper  oxide. 

Determination  of  the  Manganese,  Alumina,  Lime  and  Mag- 
nesia, Potash  and  Soda. — 100  cubic  centimetres  of  the  solution 
are  boiled  with  a  little  nitric  acid,  acid  carbonate  of  ammonia 
added  until  the  fluid  is  nearly  neutral,  and  then  to  the  dear 
red  liquid,  ammonium  acetate  in  excess  :  boil  for  some  time, 
until  the  precipitate  settles  on  removing  the  lamp.  Filter 
into  a  flask,  wash  with  water  containing  a  little  ammonium 
acetate,  dry,  ignite,  and  weigh.  The  precipitate  consists 
of  ferric  oxide,  alumina,  and  phosphoric  acid,  and  contains 
traces  of  silicic  acid  left  in  solution.  The  weighed  substance 
is  fused  with  acid-sulphate  of  potassium,  the  fused  mass 
treated  with  hot  water,  and  the  silica  filtered  off  and  weighed. 
The  amount  of  alumina  is  determined  by  subtracting  the 
total  quantity  of  ferric  oxide,  phosphoric  acid,  and  silica  from 
the  original  weight  of  the  ignited  precipitate.  •  If  it  is  re- 
quired to  determine  the  alumina  directly,  add  tartaric  acid 
to  the  solution,  then  ammonium  chloride,  and  ammonium 
sulphide.  The  precipitate  after  standing  a  few  hours  is 
filtered  off,  and  washed  with  water  containing  ammonium 
sulphide.  Add  sodium  carbonate  and  nitre  to  the  filtrate, 
evaporate  to  dryness,  ignite,  dissolve  in  dilute  hydrochloric 
acid,  filter  if  necessary,  and  precipitate  the  alumina  by  the 
addition  of  ammonium  chloride  and  ammonia  in  slight 
excess,  boil  until  the  ammonia  is  expelled,  and  wash 
thoroughly  with  hot  water.  From  the  weight  of  this  precipi- 
tate the  amount  of  phosphoric  acid  is  to  be  subtracted, 
since  it  is  precipitated  in  the  process  together  with  the 
alumina. 

The  filtrate  from  the  basic  acetates  contains  the  manganese, 
alkalies,  and  alkaline  earths.  A  few  drops  of  bromine  are 
added,  the  solution  is  heated  to  40  or  50°,  and  the  flask 
tightly  corked.  In  a  few  hours  the  whole  of  the  man- 
ganese separates  out  as  binoxide  :  it  is  filtered  off,  dried, 


Iron  Ores.  221 

ignited,  and  weighed  as  Mn3O4.  Concentrate  the  filtrate, 
add  ammonia  and  ammonium  oxalate,  and  convert  the  pre- 
cipitate into  lime  by  ignition.  The  filtrate  contains  the 
magnesia  and  alkalies  mixed  with  a  large  quantity  of 
ammonium  salts.  It  is  evaporated  to  dryness,  ignited  to 
expel  the  ammoniacal  salts,  treated  with  a  small  quantity  of 
water,  about  i  gram  of  oxalic  acid  added,  and  the  solution 
again  evaporated  to  dryness  and  ignited.  In  this  process 
the  alkalies  are  left  as  carbonates,  and  the  magnesium  chloride 
is  converted  into  magnesia.  The  residue  is  treated  with  a  little 
water,  and  the  magnesia  filtered  off,  dried,  and  weighed.  The 
carbonates  in  solution  are  converted  into  chlorides,  weighed, 
and  separated  as  in  No.  IV.  Part  II.  If  great  accuracy  is 
required,  the  nitrate  containing  the  excess  of  platinum  is 
reduced  by  means  of  hydrogen,  filtered  from  the  precipitated 
metal,  and  the  small  quantity  of  magnesia  usually  present  in 
solution  precipitated  by  the  addition  of  ammonia  and  a  drop 
or  two  of  sodium  phosphate.  (See  Analyses  of  Plant-ashes.) 

Determination  of  the  Iron  by  Solution  of  Potassium  Bichro- 
mate.— When  a  solution  of  potassium  bichromate  is  added 
to  a  liquid  containing  a  ferrous  salt  and  free  hydrochloric 
acid,  the  iron  is  converted  into  a  ferric  salt  in  accordance 
with  the  reaction  : 

6FeCl2  +  K2Cr207  +  i4HCl  =  3Fe2Cl6  +  2KC1  +  Cr2Cl6 

+  7H20. 

The  final  point  of  the  reaction — that  is,  the  point  at  which  the 
whole  of  the  iron  is  converted  into  ferric  chloride — is  ascer- 
tained by  bringing  a  drop  of  the  solution  in  contact  with 
potassium  ferricyanide,  when  no  blue  colouration  will  be 
produced.  So  long  as  the  faintest  trace  of  ferrous  chloride 
remains,  a  drop  of  the  liquid  will  colour  the  potassium  ferri- 
cyanide. It  is  evident  from  the  equation  that  i  eq.  or 
294-42  parts  of  the  bichromate  will  convert  6  eq.  or  336 
parts  of  iron  to  the  state  of  a  ferric  salt.  By  dissolving 
4-907  grams  of  pure  dry  potassium  bichromate  in  a  litre  of 


222  Quantitative  Chemical  Analysis. 

water,  a  solution  is  obtained  of  which  i  cubic  centimetre  is 
equivalent  to  "0056  gram  of  iron. 

The  potassium  bichromate  is  purified  by  recrystallisation, 
dried  between  blotting  paper,  fused,  and  roughly  powdered. 
About  5  grams  of  the  salt  are  then  weighed  out  into  a  litre 
flask,  and  diluted  with  so  much  water  that  each  cubic  centi- 
metre contains  0*004907  gram  of  the  bichromate.  Suppos- 

FIG.  58. 


ing  that  5*073  grams  have  been  weighed  out,  this  would 
require  1033-8  cubic  centimetres,  since 

4-907   :   1000  ::  5-073    :     1033-8. 

The  litre  flask  containing  the  salt  is  rilled  up  to  the  con- 
taining-mark,  and  the  33-8  cubic  centimetres  added  from  a 
burette.  If  there  is  not  sufficient  space  between  the  mark 


Iron-Ores. 


223 


and  stopper  of  the  flask,  the  33*8  cubic  centimetres  are  poured 
into  the  dry  bottle  in  which  the  solution  is  to  be  preserved, 
and  the  1,000  cubic  centimetres  of  bichromate  solution  added; 
the  liquid  is  well  shaken,  the  litre  flask  refilled  with  it,  and 
the  liquid  poured  back :  if  this  process  is  repeated  two  or 
three  times  the  solution  will  be  of  uniform  strength.  It  is 
advisable,  however,  to  test  the  strength  of  the  solution  by 
direct  experiment.  For  this  purpose  about  0*2  gram  of  fine 
pianoforte  wire  is  weighed  out  with  the  greatest  care,  and  dis- 
solved in  a  few  cubic  centimetres  of  pure  hydrochloric  acid, 
in  the  flask  a  represented  in  fig,  58.  During  the  solution  a 
slow  current  of  carbonic  acid  is  passed  through  the  apparatus; 
the  exit  tube  e  is  furnished  with  a  little  valve,  made  by 
cutting  a  short  slit  in  a  small  piece  of  caoutchouc  tubing, 
slipping  the  one  end  over  the  tube  and  stopping  the  other 
by  means  of  a  glass  rod.*  This  little  valve  opens  by  inward 
pressure  only  ;  as  soon  as  the  pressure  is  applied  outwardly, 
the  sides  of  the  slit  are  pressed  together  and  effectually 
prevent  the  entrance  of  air.  In  this  manner  any  chance  of 
the  ferrous  solution  within  the  flask  oxidising  is  prevented. 
Whilst  the  iron  is  dissolving,  make  a  solution  of  potassium 
ferricyanide  by  dissolving  a  minute  portion  of  the  salt  in 
about  15  cubic  centimetres  of  water  in  a  test-glass.  The 
solution  of  the  ferricyanide  should  be  very  dilute,  other- 
wise it  gives  a  reddish  precipitate  towards  the  completion  of 
the  assay.  Spread  a  few  small  drops  over  a  plate  or  porce- 
lain slab  by  means  of  a  glass  rod,  and  fill  up  the  burette  with 
the  solution  of  bichromate.  When  all  the  iron  is  dissolved, 
the  solution  is  boiled  for  a  minute,  and  mixed  with  a  quantity 
of  cold  water,  and  the  bichromate  is  added  to  it  with 
constant  stirring.  The  solution  in  the  flask  changes  rapidly 

*  It  is  convenient,  when  a  number  of  such  estimations  have  to  be 
made,  to  determine  the  weight  of  a  certain  length  of  the  piano-wire,  and 
to  cut  off  a  portion  .when  needed  equivalent  to  0-2  gram.  The  amount 
taken  must  of  course  be  controlled  by  weighing  the  wire.  A  milli- 
metre scale  fastened  on  to  the  balance-table  will  be  found  very  useful 
for  this  and  similar  purposes. 


224  Quantitative  CJiemical  Analysis. 

in  colour  and  becomes  dark  green.  A  brown  colour  in- 
dicates that  a  deficiency  of  hydrochloric  acid  is  present. 
From  time  to  time  a  drop  of  the  solution  is  brought  from 
the  flask  in  contact  with  the  ferricyanide  upon  the  slab. 
When  the  intensity  of  the  blue  produced  begins  to 
diminish,  the  bichromate  solution  must  be  more  slowly 
added.  The  mixed  drops  soon  acquire  a  greenish  tint,  and 
when  the  last  trace  of  this  colour  disappears  the  reaction  is 
finished.  A  slight  correction  requires  to  be  made  on  the 
quantity  of  iron  taken,  on  account  of  the  impurity  present  in 
it.  If  pianoforte  wire  be  used,  it  may  be  assumed,  without 
sensible  error,  that  it  contains  997  per  cent,  of  iron.* 

Eocample. — A  solution  of  bichromate  was  made  in  accord- 
ance with  the  directions  given:  5*073  grams  of  the  re- 
crystallised  and  fused  salt  being  dissolved  in  1033*8  cubic 
centimetres  of  water.  0*2097  gram  of  piano- wire  was 
then  dissolved  in  hydrochloric  acid.  This  solution  required 
38*0  cubic  centimetres  of  bichromate  solution  before  the 
blue  colour  disappeared :  38*0  cubic  centimetres  bi- 
chromate are  therefore  equal  to  (0-2097  x  '997)  =  '2091 
gram  of  iron,  or  i  cubic  centimetre  of  bichromate  is  equiva- 
lent to  0*0055  gram  of  iron. 

25  cubic  centimetres  of  the  solution  of  the  iron  ore  to  be 
tested  are  measured  off  into  the  flask,  a  few  cubic  centi- 
metres of  hydrochloric  acid  added,  and  two  or  three  small 
pieces  of  pure  zinc.  The  flask  is  connected  with  the  carbonic 
acid  apparatus,  and  the  evolution  of  the  hydrogen  promoted 
by  a  gentle  heat.  When  the  solution  has  become  nearly 
colourless,  or  possesses  only  a  faint  tinge  of  green,  a  minute 
drop  is  withdrawn  and  tested  with  potassium  thiocyanate, 
water  is  added,  and  the  bichromate  solution  is  poured  in 
until  the  iron  is  completely  converted  into  ferric  chloride. 

*  The  bichromate  solution  may  also  be  standardised  by  means  of 
ferrous  sulphate  precipitated  by  alcohol  (p.  150) :  the  salt  thus  purified 
may  be  preserved  in  a  stoppered  bottle  without  experiencing  the  least 
change. 


Titaniferous  Iron-Ore.  225 

The  experiment  is  repeated  with  a  second  portion  of  25 
cubic  centimetres  of  the  original  solution. 

In  ores  containing  a  mixture  of  ferrous  and  ferric  oxides, 
as  magnetic  or  spathose  iron-stones,  the  amount  of  the 
former  oxide  is  determined  by  dissolving  from  i  to  3  grams 
of  the  sample,  according  to  its  supposed  richness,  in  hydro- 
chloric acid,  in  a  stream  of  carbonic  acid,  adding  water,  and 
titrating  with  bichromate  solution  according  to  the  method 
just  given. 

XX.    TITANIFEROUS  IRON-ORE  (ILMENITE). 

Ilmenite  is  a  naturally  occurring  ferrous  titanate  (FeTiO3), 
but  it  is  seldom  met  with  quite  pure  ;  it  usually  contains 
ferric  oxide,  manganous  oxide,  magnesia,  alumina,  silica,  &c. 
Many  iron-sands  contain  considerable  quantities  of  ilmenite, 
associated  with  magnetic  oxide  of  iron.  As  such  ores  are 
occasionally  used  in  the  manufacture  of  iron,  it  may  be 
desirable  to  give  a  method  for  their  analysis. 

The  weighed  portion  of  the  mineral,  in  a  state  of  fine 
powder,  is  fused  with  about  eight  times  its  weight  of  acid 
sodium  sulphate,  and  on  cooling  the  fused  mass  is  digested 
with  cold  water.  The  insoluble  matter  is  filtered  off,  washed, 
dried,  and  weighed  :  it  is  usually  free  from  titanic  acid.  It 
should,  however,  be  treated  by  the  method  given  in  No.  XIX., 
p.  218.  Dilute  the  filtrate  considerably,  add  a  little  nitric 
acid  to  it,  and  boil  for  some  time  to  precipitate  the  titanic 
acid.  Filter  the  precipitate,  wash,  dry,  and  ignite  it,  with  the 
addition  of  a  fragment  of  ammonium  carbonate,  to  ensure  the 
volatilisation  of  sulphuric  acid,  which  the  precipitate  is  apt 
to  carry  down  with  it.  Titanic  acid  is  slightly  hygroscopic : 
it  must  be  weighed,  therefore,  as  expeditiously  as  possible. 
The  iron,  magnesia,  lime,  and  alumina  remain  in  solution,  and 
may  be  separated  by  the  methods  given  in  No.  XIX.,  p.  217. 

Titaniferous  ores  may  also  be  decomposed  by  heating 
with  sulphuric  or  hydrochloric  acid,  under  pressure.  A  rapid 
and  accurate  method  of  estimating  iron  and  titanium  when 
present  in  solution  together,  after  fusing  the  ore  with  acid 

Q 


226  Quantitative  Chemical  Analysis. 

sodium  sulphate,  and  treating  the  fused  mass  with  water, 
is  founded  on  the  behaviour  of  these  substances,  when 
reduced  to  their  lowest  state  of  oxidation,  towards  a  solu- 
tion of  potassium  permanganate.  The  solution  thus  ob- 
tained is  diluted  to  a  determinate  amount,  an  aliquot 
portion  withdrawn,  and  reduced  by  zinc  and  sulphuric  acid 
in  the  apparatus  represented  in  fig.  58  (p.  222).  The  iron 
and  titanic  acid  are  thus  brought  to  the  state  of  ferrous  and 
titanous  oxides,  and  their  joint  amount  is  determined  by 
addition  of  standard  permanganate  solution  until  the  rose- 
colour  of  the  latter  solution  is  permanent.  A  second  aliquot 
portion  is  brought  into  the  flask,  and  an  apparatus  for  the 
disengagement  of  sulphuretted  hydrogen  or  sulphur  dioxide 
is  substituted  for  the  carbonic  acid  flask.  The  iron  is  thus 
reduced  to  the  ferrous  state,  the  titanic  acid  remains  un- 
changed. The  solution  is  boiled  to  expel  the  excess  of  the 
reducing  agent,  whereby  a  portion  of  the  titanic  acid  is 
precipitated,  filtered  rapidly,  and  the  solution  is  again  titrated 
with  permanganate.  The  amount  of  ferrous  oxide  is  thus  ob- 
tained :  the  difference  between  the  titrations  gives  the  amount 
of  permanganate  corresponding  to  the  titanic  acid  present. 
The  weight  of  the  latter  substance  is  found  from  the  equation 

Ti2O3  +  O     =     2TiO2. 

1 6  parts  of  oxygen  are  therefore  equivalent  to  164  parts  of 
titanic  oxide. 

This  process  is  also  applicable  to  the  analysis  of  rutile 
(native  titanic  oxide),  sphene  or  titanite  (a  silico-titanate  of 
calcium,  CaSiO3.CaTiO3),  &c. 

XXI.  WROUGHT  AND  CAST  IRON,  AND  STEEL. 
Cast  iron,  in  addition  to  chemically-combined  carbon  and 
graphite,  contains  variable  quantities  of  silicium,  phosphorus, 
sulphur,  manganese,  and  copper  ;  and  in  very  much  smaller 
quantities,  aluminium,  chromium,  titanium,  zinc,  nickel,  cobalt, 
arsenic,  antimony,  tin,  vanadium,  magnesium,  potassium, 
lithium,  sodium,  and  nitrogen.  The  greater  number  of  these 


Wrought  and  Cast  Iron,  &c. 


227 


substances  are  present  in  such  very  minute  quantities,  that 
their  exact  quantitative  determination  is  a  matter  of  great 
difficulty.  It  is  very  rarely  necessary  to  attempt  their  esti- 
mation, since  their  presence  in  such  small  amount  probably 
exercises  little  or  no  influence  on  the  quality  of  the  iron. 
Cast  iron  occurs  in  two  leading  varieties,  viz.  as  grey  and  as 
white  cast  iron.  The  difference  between  the  varieties  is 
determined  by  the  state  of  the  carbon  present  in  them.  In 
grey  cast  iron  the  greater  amount  of  the  carbon  is  in  the  form 
of  graphite,  interspersed  throughout  the  mass  in  a  state  of 
mechanical  mixture  ;  in  the  white  variety  the  carbon  is 
chemically  combined  with  the  iron.  Intermediate  varieties, 
in  which  grey  and  white  cast  iron  are  mixed  together  in 
varying  proportions,  are  classed  as  mottled  cast  iron. 

The  following  analyses  of  grey  and  white  cast  iron  obtained 
from  the  same  ore  (spathic,  containing  about  42  per  cent. 
iron)  clearly  show  this  characteristic  difference  :  — 

Grey.  White. 

Iron 

Combined  Carbon 
Graphite. 
Silicon    . 
Sulphur  . 
Phosphorus 
Manganese 
Xickel  and  Cobalt 

Wrought  iron  approaches  more  nearly  to  the  character  of 
pure  iron  :  it  contains  much  less  carbon  than  cast  iron,  melts 
at  a  higher  temperature,  and  is  heavier.  It  also  contains 
much  less  silicon,  but  the  amounts  of  sulphur  and  phosphorus 
are  as  variable  as  in  cast  iron.  The  subjoined  analysis  is  of 
Swedish  iron  of  excellent  quality  :  — 


90-584 

93-I83 

— 

4'  100 

2795 

— 

4-414 

0-230 

0-039 

0-030 

0-099 

0-073 

1-837 

2-370 

traces 

0-014 

99768 

loo-ooo 

Iron  . 
Carbon  . 
Silicon  . 
Sulphur  . 
Phosphorus 


99*863 
o-o54 

O-O28 

o-o55 

traces 

loo-ooo 


Q  2 


228  Quantitative  Chemical  Analysis. 

Steel  is  intermediate  in  properties  and  purity  between 
cast  iron  and  wrought  iron  :  it  differs,  however,  from  these 
varieties  by  its  remarkable  and  valuable  property  of  becom- 
ing hardened  by  heating  and  sudden  cooling.  In  this  state 
it  is  extremely  brittle,  and  is  without  the  characteristic  fibre 
of  malleable  iron.  Its  tenacity,  however,  is  much  greater 
than  that  of  wrought  iron.  Its  average  specific  gravity  is 
intermediate  between  that  of  cast  iron  and  wrought  iron. 
The  following  is  an  analysis  of  steel  of  medium  quality  : — 


Carbon    . 

Silicon    . 

Sulphur  . 

Phosphorus 

Manganese 

Iron  (by  difference) . 


0-501 
0-106 
0-002 
0-096 
0-144 


100-000 


The  action  of  acids  upon  these  varieties  of  iron  is  remark- 
able. When  heated  gently  with  strong  hydrochloric  acid 
white  cast  iron  is  completely  dissolved,  whereas  grey  cast 
iron  leaves  a  residue  of  graphite.  The  combined  carbon 
present  in  both  varieties  combines  with  the  nascent  hydrogen 
evolved  by  the  action  of  the  acid,  giving  rise  to  hydrocarbons 
of  the  CnH2n  series,  C2H4  .  .  .  C6H12,  which  commu- 
nicate a  peculiar  odour  to  the  issuing  gas. 

The  diluted  acid  at  ordinary  temperatures  attacks  cast 
iron  but  slowly  :  when  dissolved  by  the  aid  of  heat,  white 
cast  iron  deposits  a  portion  of  its  combined  carbon  ;  this  is 
soluble  in  potash,  and  on  ignition  leaves  a  black  residue, 
containing  silica.  The  residue  from  grey  cast  iron,  in  addition 
to  graphite,  also  contains  this  carbonaceous  matter  together 
with  a  black  magnetic  substance  containing  iron.  When 
dried  this  residue  occasionally  takes  fire  in  contact  with 
oxygen,  and  is  converted  into  a  mixture  of  ferric  oxide  and 
silica. 

It  is  said  that  the  action  of  acids  upon  steel  varies  with  its 
hardness :  soft  steel  is  far  more  readily  dissolved  than  hardened 


Wrought  and  Cast  Iron,  &c.  229 

steel.  Concentrated  hydrochloric  acid  dissolves  soft  steel 
completely,  but  when  diluted  it  leaves  a  larger  amount  of  the 
black  magnetic  substance  above  mentioned  than  is  yielded 
by  malleable  iron.  It  is  worthy  of  note  that  diluted  nitric 
acid  completely  dissolves  this  carbonaceous  matter,  forming 
a  yellowish-brown  coloured  liquid,  the  intensity  of  the  colour 
being  proportional  to  the  amount  of  combined  carbon  present. 
It  is  said  that  steel  may  be  distinguished  from  cast  and 
wrought  iron  by  the  action  of  hydrochloric  acid  of  sp.gr.  1*134. 
With  steel  the  acid  occasions  a  rapid  evolution  of  gas,  which 
suddenly  ceases  in  a  short  time  (in  about  20  seconds), 
whereas  with  cast  or  wrought  iron  the  disengagement  is  con- 
tinuous. 

The  complete  analysis  of  iron,  in  addition  to  the  estima- 
tion of  the  metal,  necessitates  the  determination  of  the 
carbon,  existing  both  in  a  state  of  combination  and  as 
graphite,  of  silicon,  sulphur,  manganese  (zinc,  cobalt, 
alumina,  titanic  acid),  phosphorus,  nitrogen,  and  admixed 
slag.  The  iron,  however,  is  usually  determined  by  dif- 
ference. 

Determination  of  the  Total  Carbon. — Among  the  many 
accurate  methods  which  have  been  proposed  for  the  estima- 
tion of  the  total  carbon  contained  in  iron,  those  of  Wohler, 
Weyl,  and  Ullgren  are  distinguished  by  reason  of  the  ease 
and  expedition  with  which  they  may  be  carried  out.  In  all 
these  processes  the  carbon  is  ultimately  weighed  as  carbon 
dioxide. 

(a)  Wohler 's  Method. — By  burning  the  iron  in  a  stream  of 
oxygen.  The  iron  must  be  previously  reduced  to  the  finest 
possible  state  of  division,  by  filing  with  a  hard  file,  and 
powdering  in  a  large  agate  mortar.  If  the  metal  is  very 
hard  it  is  broken  on  a  clean  anvil,  stamped  to  powder  in  the 
steel  mortar,  and  passed  through  a  fine  sieve.  From  3  to  6 
grams  of  the  metal  (according  to  its  supposed  richness  in 
carbon)  are  weighed  out  into  a  platinum  boat,  and  brought 


Wrought  and  Cast  Iron,  &c.  231 

into  a  short  piece  of  combustion  tubing  drawn  out  at  one 
end  to  a  narrow  tube  (fig.  59).  The  other  end  is  closed 
with  a  caoutchouc  cork,  and  is  connected  with  a  gasometer 
filled  with  oxygen.  To  the  narrowed  end  of  the  combustion 
tube  is  attached  a  chloride  of  calcium  tube,  connected  with 
a  weighed  U-tube  filled  -J  with  soda-lime,  and  -J-  with  calcium 
chloride  (fig.  60).  A  is  a  gasometer  containing  oxygen ;  by 
means  of  the  cock  s  the  amount  of  the  issuing  gas  can  be 
easily  regulated.  The  rate  of  its  passage  can  be  seen  in  b,  which 
contains  solution  of  caustic  potash,  destined  to  absorb  any 
traces  of  carbonic  acid  and  chlorine  which  may  be  present 
in  the  oxygen.  The  cylinder  c  is  partially  filled  with  soda- 
lime  with  the  same  object :  the  upper  half  and  the  two  U- 
tubes  contain  calcium  chloride,  by  which  the  gas  is  thoroughly 
dried.  The  combustion  tube  containing  the  weighed  amount 
of  iron  rests  in  a  gas  furnace  j  d  is  a  chloride  of  calcium 
tube  :  it  is  placed  merely  as  a  precaution  against  moisture 
passing  into  the  weighed  U-tube,  and  need  not  be  weighed. 
The  little  U-tube /con  tains  one  or  two  drops  of  concentrated 
sulphuric  acid,  sufficient  to  fill  the  bend ;  its  object  is  to 
prevent  the  possibility  of  the  calcium  chloride  in  the  weighed 
tube  absorbing  atmospheric  moisture.  The  process  of  com- 
bustion needs  no  particular  attention ;  if  the  heat  is  sufficiently 
powerful  and  the  iron  finely-divided,  the  whole  of  the  carbon 
is  converted  into  carbon  dioxide.  When  the  process  is 
finished,  b  is  detached  from  the  gasometer,  and  a  slow  current 
of  air  is  aspirated  through  the  apparatus  to  expel  the  oxygen 
before  the  U-tube  is  again  weighed. 

(b)  Weyl's  Method* — In  pulverising  the  iron  for  analysis, 
especially  if  very  hard,  there  is  considerable  risk  of  mixing 
the  sample  with  iron  from  the  file,  &c.,  employed.  Weyl's 
method,  by  dispensing  with  the  necessity  of  reducing  the 
iron  to  powder,  obviates  this  inconvenience.  A  piece  of  the 

*  This  method  appears  to  have  been  described  so  far  back  as  1857 
by  Binks,  in  a  paper  read  before  the  Society  of  Arts. 


232  Quantitative  Chemical  Analysis. 

iron  weighing  from  10  to  15  grams  is  suspended  by  a  pair  of 
pincettes  provided  with  platinum  points  in  a  beaker  con- 
taining dilute  hydrochloric  acid.  Care  must  be  taken  that  the 
platinum  points  in  contact  with  the  iron  are  not  moistened 
with  the  acid,  or  its  solvent  action  will  be  impeded 
from  the  separation  of  carbon  between  the  points  and  the 
metal.  Connect  the  upper  portion  of  the  pincettes  with  the 
wire  of  a  positive  pole  of  a  single  element  of  Bunsen's  battery. 
To  the  wire  of  the  negative  pole  is  attached  a  slip  of  plati- 
num foil,  which  is  also  immersed  in  the  liquid.  By  regulating 
the  distance  between  the  foil  and  the  metal,  the  strength  of 
the  current  may  be  so  modified  that  not  a  trace  of  ferric 
chloride  is  produced.  The  due  regulation  of  the  intensity 
is  of  the  utmost  importance,  for  if  it  is  too  strong  the  iron 
becomes  passive,  and  chlorine  is  evolved  from  its  surface, 
which  brings  about  the  oxidation  of  any  separated  carbon. 
This  formation  of  chlorine  is  immediately  rendered  evident 
by  the  yellowish  colour  of  the  concentrated  solution  of  iron  as 
it  falls  away  from  the  metal.  When  the  operation  succeeds, 
hydrogen  only  appears  at  the  platinum  foil,  no  gas  being 
evolved  from  the  positive  pole  (the  iron).  In  12  or 
15  hours  the  whole  of  the  iron  immersed  will  be  dissolved  : 
the  separated  carbon  retains  the  shape  of  the  metal.  The 
undissolved  portion  is  detached  from  the  spongy  mass  of 
carbon,  dried,  and  weighed.  Its  weight  subtracted  from  that 
of  the  iron  originally  taken  gives  the  amount  employed  for 
the  carbon  determination.*  The  carbon  is  washed  slightly, 
and  thrown  into  a  short  piece  of  combustion  tube  containing 
a  plug  of  ignited  asbestos.  This  acts  as  a  filter  and  retains 
the  carbon  :  care  must  of  course  be  taken  that  the  plug  is 
sufficiently  compact  to  prevent  any  particles  of  carbon 
passing  through  with  the  filtrate.  The  tube  is  gently  heated 

*  Instead  of  suspending  the  iron  by  the  pincettes  as  above  described, 
it  may  be  broken  into  several  pieces,  and  supported  on  a  small  platinum 
tray  pierced  with  a  number  of  holes.  In  this  way  the  whole  of  the  iron 
may  be  dissolved. 


Wrought  and  Cast  Iron,  &c.  233 

and  a  current  of  air  aspirated  through  it.  When  perfectly 
freed  from  moisture,  the  plug  together  with  the  carbonaceous 
residue,  is  drawn  by  means  of  a  bent  wire  into  the  middle  of 
the  tube,  and  is  mixed  with  a  small  quantity  of  copper  oxide 
by  the  aid  of  the  wire.  The  combustion  tube  is  heated  in  a 
gas  furnace,  and  the  carbon  dioxide  collected  in  a  weighed 
soda-lime  tube  as  described  above.  To  ensure  perfect  com- 
bustion it  is  advisable  to  heat  the  carbon  and  copper  oxide 
in  a  stream  of  oxygen  :  for  this  purpose  the  apparatus  repre- 
sented in  fig.  60  may  be  employed.  Instead  of  burning  it, 
the  separated  carbon  may  be  treated  as  in  the  next  method. 

(c)  Ullgren's  Method. — It  is  necessary  for  this  method  that 
the  sample  should  be  in  a  state  of  coarse  powder.  Grey  cast 
iron  is  preferably  taken  in  the  form  of  borings  :  white  cast 
iron  should  be  coarsely  pondered.  Dissolve  10  grams  of 
copper  sulphate  in  about  50  cubic  centimetres  of  water,  and 
into  this  solution,  contained  in  a  small  beaker,  weigh  out  about 
2  grams  of  the  iron.  Heat  gently  and  with  constant  stirring 
until  the  iron  is  completely  dissolved ;  allow  the  solid  portions 
to  settle  and  pour  away  as  much  as  possible  of  the  clear 
fluid.  Rinse  the  solid  particles  with  any  adhering  copper 
solution  into  the  small  flask  A  (fig.  31),  which  should  have  a 
capacity  of  about  150  cubic  centimetres.  Care  must  be 
taken  not  to  employ  too  large  a  quantity  of  wash  water  in 
rinsing  the  reduced  copper  and  carbon  into  the  flask :  the 
total  fluid  in  the  flask  should  not  exceed  25  cubic  centimetres. 
Add  to  the  flask  40  cubic  centimetres  of  concentrated  sul- 
phuric acid  :  if  more  than  25  cubic  centimetres  of  wash 
water  have  been  used,  proportionally  more  acid  must  be  em- 
ployed. Allow  the  mixture  to  cool,  add  about  8  grams 
of  chromic  acid,  and  connect  the  flask  with  the  system  of 
tubes  represented  in  fig.  31.  Heat  the  liquid  gradually,  and 
regulate  the  flame  so  as  to  maintain  a  regular  evolution  of 
gas  :  as  it  slackens,  increase  the  heat,  until  white  fumes  make 
their  appearance  in  the  body  of  the  flask.  Remove  the  lamp 


2 34  Quantitative  Chemical  Analysis. 

and  aspirate  a  slow  current  of  air  through  the  apparatus  (3 
or  4  litres).  Weigh  the  soda-lime  tube  and  again  aspirate 
air,  in  order  to  determine  if  the  increase  of  weight  due  to  the 
absorption  of  the  carbon  dioxide  is  constant.  The  prin- 
ciple of  the  method  is  evident.  In  contact  with  the  sul- 
phuric and  chromic  acids  the  carbon  separated  on  dissolving 
the  iron  is  converted  into  carbon  dioxide. 

Note  on  the  preparation  of  Chromic  Acid. — 300  grams  of  coarsely- 
powdered  commercial  potassium  bichromate  are  warmed  with  500  cubic 
centimetres  of  water  and  420  cubic  centimetres  of  sulphuric  acid.  When 
the  salt  is  dissolved  the  solution  is  allowed  to  stand  ten  or  twelve  hours, 
when  the  acid  potassium  sulphate  separates  out.  The  mother  liquor  is 
decanted,  and  the  crystals  allowed  to  drain  for  an  hour.  The  solution 
is  heated  to  80°  or  90°,  and  gradually  mixed  with  1 50  cubic  centimetres 
of  sulphuric  acid,  and  then  with  the  same  quantity  of  water,  when  the 
precipitated  chromic  acid  will  be  redissolved.  The  solution  is  concen- 
trated in  a  porcelain  basin  until  small  spicular  crystals  appear  on  the 
surface  of  the  liquid  :  after  standing  a  few  hours  an  abundant  crop  of 
chromic  acid  crystals  will  be  obtained.  The  mother  liquor  on  further 
evaporation  will  yield  a  fresh  quantity  of  crystals.  The  chromic  acid 
thus  obtained  may  be  drained  by  means  of  the  filter  pump,  and  dried 
on  a  porous  tile  placed  beneath  a  bell  jar :  it  is  very  hygroscopic  and 
must  be  kept  in  a  well-stoppered  bottle.  The  crystals  are  not  quite 
pure,  as  they  contain  small  quantities  of  potash  and  sulphuric  acid,  but 
the  presence  of  these  substances  in  no  way  interferes  with  their  employ- 
ment in  the  above  method. 

Instead  of  absorbing  the  liberated  carbonic  acid  by  means 
of  soda-lime,  Ullgren  prefers  to  use  potash-pumice.  This  is 
prepared  as  follows  : — i  part  of  caustic  potash  is  dissolved  in 
3  or  4  parts  of  water,  the  solution  is  heated  to  100°  in  an 
iron  pot,  and  a  quantity  of  pumice,  in  pieces  somewhat  less 
than  the  size  of  a  pea,  is  added  until  the  mass  becomes  nearly 
dry.  Whilst  still  hot  it  is  transferred  to  a  wide-mouthed 
stoppered  bottle,  and  briskly  shaken  until,  on  cooling,  the 
small  pieces  no  longer  adhere  to  each  other. 

Determination  of  the  Graphite. — The  above  methods,  it 
will  be  observed,  determine  merely  the  total  amount  of  carbon, 
and  give  us  no  information  respecting  the  proportion  present 
as  graphite  and  as  combined  carbon.  In  order  to  determine 


Wrought  and  Cast  Iron,  &c.  235 

the  graphite,  3  to  5  grams  of  the  sample  are  dissolved  in 
moderately-concentrated  hydrochloric  acid  at  a  gentle  heat 
(comp.  p.  228).  The  combined  carbon  combines  with  the 
nascent  hydrogen,  and  is  evolved  together  with  the  excess  of 
this  gas :  the  graphite  remains  undissolved.  Filter  the 
residue  through  asbestos,  contained  in  a  short  piece  of  com- 
bustion tube  (fig.  59,  p.  230),  wash  it  with  hot  water,  then 
with  potash,  alcohol,  and  a  little  ether.  Dry  it,  mix  it  with 
a  little  copper  oxide,  and  burn  it  in  a  stream  of  oxygen  as 
directed  on  p.  233  (see  fig.  60).  On  deducting  the  weight  of 
the  graphite  thus  obtained  from  the  total  amount  of  the  car- 
bon, the  difference  gives  the  quantity  of  combined  carbon. 
This  process  is  the  most  uniformly  accurate,  but  for  technical 
purposes  it  may  be  thus  simplified.  Treat  the  weighed  sample 
of  iron  with  dilute  hydrochloric  acid,  and  when  the  metal  is 
nearly  dissolved,  add  a  large  quantity  of  strong  hydrochloric 
acid.  The  insoluble  matter  is  collected  on  a  weighed  filter, 
washed  with  hot  water,  with  dilute  hydrochloric  acid  to  re- 
move iron,  and  with  potash  to  remove  silica.  In  washing  with 
the  alkali  there  is  occasionally  a  brisk  evolution  of  hydrogen, 
owing  to  the  oxidation  of  the  silicon  to  silicic  acid.  The 
insoluble  matter  is  lastly  washed  with  alcohol  and  ether  to 
remove  any  traces  of  adhering  hydrocarbons,  dried  at  120° 
and  weighed.  The  filter  is  thrown  into  a  small  platinum 
crucible,  and  incinerated  :  the  weight  of  the  residue,  less  that 
of  the  filter-ash,  gives  the  amount  of  silica  (and  titanic  acid) 
mixed  with  the  graphite. 

Estimation  of  Combined  Carbon  in  Steel  and  Wrought  Iron. 

Eggertz's  Method. — In  the  case  of  metallic  iron  containing 
but  a  small  proportion  of  combined  carbon,  this  method  is 
readily  applicable.  When  such  iron  is  dissolved  in  nitric 
acid,  the  solution  becomes  coloured  more  or  less  brown  in 
proportion  to  the  amount  of  carbon  present  (comp.  p.  229). 
By  comparing  the  depth  of  this  coloration  with  that  of  a 
standard  tint,  equivalent  to  a  known  quantity  of  carbon,  the 


236  Quantitative  Chemical  Analysis. 

determination  of  the  combined  carbon  in  a  sample  of  steel 
or  wrought  iron  may  be  made  in  a  very  short  time,  and  with 
great  accuracy.  The  process  is  therefore  especially  appli- 
cable in  steel  works,  where  such  determinations  are  of  frequent 
occurrence.  It  is  thus  conducted.  Two  thin  test-tubes, 
made  of  the  same  glass,  are  divided  into  0-5  cubic  centimetre 
by  means  of  a  pipette,  the  graduation  being  scratched  on  the 
side  by  a  diamond.  A  piece  of  steel  weighing  not  less  than 
100  grams,  supposed  to  contain  from  07  to  ro  per  cent,  of 
carbon,  is  finely  powdered,  and  the  amount  of  carbon  it  con- 
tains determined  by  one  of  the  methods  above  described. 
As  this  sample  is  to  serve  as  the  standard  of  comparison, 
the  amount  of  carbon  which  it  contains  must  be  estimated 
with  the  utmost  possible  accuracy :  it  is  advisable  therefore 
to  make  several  determinations,  and  to  take  the  mean  of  the 
results.  Supposing,  for  the  sake  of  illustration,  that  the 
sample  contains  075  per  cent,  carbon.  One  decigram  of 
the  steel  or  wrought  iron  to  be  tested,  and  one  decigram  of 
the  standard  steel  (containing  075  per  cent  carbon),  are 
weighed  out  with  the  greatest  accuracy  on  small  tared  watch- 
glasses.  The  samples  are  brought  into  thin  dry  test-tubes, 
and  covered  with  about  2  cubic  centimetres  of  dilute  nitric 
acid  (sp.  gr.  1*2),  perfectly  free  from  chlorine.  In  a  few 
minutes  the  greater  portion  of  the  metals  will  be  dissolved. 
The  tubes  are  placed  in  a  beaker  containing  water  at  80°, 
which  is  maintained  at  this  temperature  until  all  action  on 
the  metals  is  at  an  end.  In  the  case  of  steels  this  ceases  in 
about  two  hours.  Allow  the  tubes  to  cool,  and  decant  the 
clear  supernatant  liquid  from  the  undissolved  matter  into  the 
graduated  test-tubes ;  the  solution  of  the  standard  steel  is 
poured  into  the  one  tube,  that  of  the  iron  into  the  other. 
To  each  portion  of  residue  add  two  or  three  drops  of  nitric 
acid,  and  heat  gently  over  the  lamp  :  if  no  further  evolution 
of  gas  occurs,  the  insoluble  matter  consists  merely  of  silica  or 
graphite.  Allow  these  solutions  to  cool,  and  add  them  to 
the  contents  of  the  graduated  tubes.  Dilute  the  solution 


Wrought  and  Cast  Iron,  &c.  237 

of  the  standard  steel  until  it  exactly  measures  7-5  cubic 
centimetres :  each  cubic  centimetre  is  equivalent  therefore 
to  o'oooi  gram  of  carbon.  Now  add  water,  drop  by  drop, 
to  the  liquid  contained  in  the  other  tube,  agitate  the 
mixture,  and  compare  the  tint  with  that  of  the  liquid  in 
the  standard  tube.  If  the  tints  are  equal  read  off  the 
volume  of  the  liquid  :  each  cubic  centimetre  represents  one 
tenth  per  cent  of  combined  carbon.  Thus,  supposing 
that  on  diluting  the  liquid  to  be  tested  to  4-5  cubic  centi- 
metres it  gave  a  depth  of  colour  equivalent  to  that  of  the 
standard  solution,  then  the  sample  contains  0-45  per  cent, 
carbon.  It  must  be  remembered  that  this  method  is 
strictly  comparative  :  to  ensure  accuracy  the  circumstances 
of  temperature,  time,  amount  of  acid  used,  &c.,  must  be  as 
nearly  as  possible  identical.  The  normal  solution  must 
be  prepared  afresh  for  each  series  of  comparisons,  as  it 
gradually  becomes  paler  on  keeping,  especially  if  exposed 
to  light 

When  such  determinations  of  carbon  in  steel  or  wrought 
iron  are  of  frequent  occurrence,  as  in  iron  works,  it  is  more 
convenient  to  prepare  a  number  of  tubes  containing  the 
brown  solution,  corresponding  to  different  percentages  of 
carbon.  The  coloured  solutions  may  be  readily  obtained  by 
digesting  roasted  coffee  in  dilute  spirit :  if  kept  in  the  dark 
when  not  in  use  they  maintain  their  intensity  of  colour  un- 
impaired for  a  long  time.  Fig.  6 1  represents  a  wooden  frame 
containing  the  tubes  :  these  are  about  f  of  an  inch  in  dia- 
meter, and  about  3^-  to  4  inches  long.  The  tubes  after  the 
introduction  of  the  properly-diluted  solutions  are  hermetically 
sealed.  The  liquid  in  the  tube  placed  in  the  second  hole 
in  the  rack  is  made  to  correspond  to  the  tint  of  a  solution 
of  i  gram  of  steel  containing  0*02  per  cent  of  combined 
carbon  in  15  cubic  centimetres  of  nitric  acid  of  sp.  gr. 
i -20.  The  solution  in  the  tube  in  the  fourth  hole  cor- 
responds to  that  of  the  same  quantity  of  iron  containing  0*04 
per  cent  carbon;  and  so  on  in  regular  succession,  each  tube 


238  Quantitative  Chemical  Analysis. 

"increasing  in  value  by  0*02  per  cent.  The  last  tube  is  equiva- 
lent therefore  to  0-3  per  cent.  The  process  is  thus  conducted ; 


FIG.  61. 


i  gram  of  the  iron  or  steel  to  be  tested,  in  the  state  of  fine 
powder,  is  weighed  out  into  a  large  test-tube,  and  digested  at 
70°  or  80°  with  10  cubic  centimetres  of  the  dilute  nitric  acid, 
free  from  chlorine,  for  about  half  an  hour.  The  solution  is 
quickly  cooled  by  immersing  the  tube  in  cold  water,  and  is 
filtered,  without  the  residue  being  disturbed,  through  a  small 
dry  filter  into  a  test-tube  of  the  same  size  and  made  of  the 
same  glass  as  those  containing  the  standard  solutions.  The 
insoluble  matter  is  then  treated  with  5  cubic  centimetres  of 
the  nitric  acid,  heated  gently,  and  the  solution  added  to  the 
main  portion.  The  entire  solution  is  mixed  by  shaking,  and 
its  colour  compared  :  the  holes  in  the  stand  allow  the  tube 
to  be  placed  side  by  side  with  the  standard  solution  :  the 
number  affixed  to  the  tube  with  which  it  corresponds  in 
colour  indicates  the  percentage  amount  of  carbon  in  the 
sample.  If  the  steel  or  wrought  iron  contains  more  than  0-3 
per  cent  of  carbon,  0*5  gram  is  taken,  or  the  solution  is 
diluted  with  an  equal  volume  of  water,  shaken,  and  half  of 
it  poured  away.  The  comparison  is  assisted  by  placing  a 
white  screen  of  paper  behind  the  tubes.  In  this  manner  a 
carbon  determination  may  be  made  to  within  -01  per  cent 

Estimation  of  the  Sulphur. — When  iron  containing  sulphur 
is  treated  with  hydrochloric  acid,  the  whole  of  the  sulphur  is 
evolved  as  sulphuretted  hydrogen.  By  passing  the  sulphu- 
retted hydrogen  into  a  solution  of  bromine  in  hydrochloric 
acid,  it  is  completely  absorbed  and  converted  into  sulphuric 
acid  ;  by  converting  the  sulphuric  acid  into  barium  sulphate, 


Wrought  and  Cast  Iron,  &c. 


239 


the  amount  of  sulphur  may  be  readily  determined.  The 
apparatus  required  for  the  purpose  is  seen  in  fig.  62.  The 
flask  of  150  cubic  centimetres  capacity  is  fitted  with  a 
caoutchouc  cork,  which  has  previously  been  boiled  in  dilute 
caustic  soda  to  remove  the  sulphur  contained  in  it :  the 
cork  is  pierced  with  two  holes,  into  one  of  which  fits  a 
straight  piece  of  tubing,  curved  at  its  lower  extremity :  near 


FIG.  62. 


the  upper  end  is  a  bulb  fille^  with  dilute  hydrochloric  acid  r 
the  tube  is  closed  by  means  of  a  small  piece  of  clean  caout- 
chouc tubing  and  a  clamp.  The  second  hole  of  the  cork 
contains  a  tube  bent  at  right  angles,  to  which  is  adapted  a 
small  U-tube,  containing  the  solution  of  bromine.  It  will 
be  seen  by  the  arrangement  of  the  glass  tubes  that  the  gas 
passes  twice  through  the  liquid  contained  in  the  U-tube. 


240  Quantitative  Chemical  A  nalysis. 

Into  the  flask  weigh  out  10  or  15  grams  of  the  finely  divided 
iron,  fill  the  bulb  with  moderately  diluted  hydrochloric  acid, 
insert  the  cork,  open  the  clamp,  and  by  aspiration  at  the 
exit-tube  of  the  absorption  apparatus  bring  the  acid  into  the 
flask.  Heat  the  flask  gently,  and  introduce  fresh  acid  from 
time  to  time  until  the  iron  is  completely  dissolved.  Con- 
nect the  U-tube  with  an  aspirator,  and  gently  draw  a  current 
of  air  through  the  apparatus  to  remove  the  last  traces  of 
sulphuretted  hydrogen  in  the  flask.  Transfer  the  liquid 
from  the  U-tube  to  a  beaker,  boil  to  expel  excess  of  bromine, 
and  add  barium  chloride. 

it  is  always  advisable  to  test  the  solution  of  ferrous  chloride 
for  sulphuric  acid  by  concentrating  it,  and  adding  one  or  two 
drops  of  barium  chloride  :  if  any  precipitate  is  thus  obtained  it 
is  to  be  filtered  off,  washed,  dried,  and  weighed.  The  insoluble 
residue  should  also  be  tested  for  sulphur  by  fusing  it  with 
nitre  and  sodium  carbonate,  dissolving  in  water,  and  testing 
the  acidified  solution  with  barium  chloride.  Usually,  how- 
ever, the  residue  will  be  found  to  be  quite  free  from  this 
substance.  A  very  convenient  method  of  converting 
the  sulphur  in  iron  to  the  state  of  sulphuric  acid  consists 
in  absorbing  the  sulphuretted  hydrogen  in  a  dilute  solution 
of  potash  or  soda  free  from  sulphate.  Decant  the  alkaline 
solution  into  a  beaker,  add  a  few  drops  of  bromine  free 
from  sulphuric  acid,  heat  gently,  acidulate  with  hydro- 
chloric acid,  boil,  add  a  few  drops  of  barium  chloride,  and 
after  standing  about  twenty-four  hours  filter  off  the  precipi- 
tated barium  sulphate. 

The  sulphuretted  hydrogen  may  also  be  absorbed  by  a 
dilute  solution  of  a  cadmium  salt :  the  cadmium  sulphide 
possesses  the  advantage  that  it  can  be  dried  at  100°  without 
alteration. 

When  a  number  of  estimations  of  sulphur  in  iron  have  to  be 
made,  it  is  convenient  to  employ  a  volumetric  method  to 
determine  the  sulphuretted  hydrogen  evolved.  Both  the 
following  plans  will  be  found  to  give  concordant  results. 
The  U-tube  (fig.  62)  is  filled  with  a  solution  of  caustic  soda 


Wrought  and  Cast  Iron,  &c.  241 

(  i  of  soda  to  5  of  water)  to  absorb  the  sulphuretted  hydrogen. 
The  operation  of  dissolving  the  iron  is  made  exactly  as  in  the 
foregoing  methods.  When  the  action  is  at  an  end,  pour  the 
contents  of  the  U-tube  into  a  large  beaker,  dilute  with  about 
200  cubic  centimetres  of  boiled  water,  acidify  with  hydro- 
chloric acid,  add  a  little  starch  paste,  and  add  standard 
iodine  solution  until  the  solution  is  turned  blue. 

Or  wash  the  alkaline  solution  into  a  small  beaker 
containing  water,  to  which  a  measured  quantity,  say  10 
cubic  centimetres,  of  deci-normal  arsenious  acid  solution 
is  added.  Add  hydrochloric  acid  to  distinct  acid  reaction, 
set  aside  for  a  few  hours  in  a  warm  place,  dilute  to  a  de- 
terminate quantity,  say  100  cubic  centimetres,  filter  through 
a  dry  filter,  withdraw  25  cubic  centimetres  of  the  filtrate, 
neutralise  with  sodium  bicarbonate,  add  a  small  quantity  of 
starch  liquor,  and  a  dilute  solution  of  iodine  from  a  burette 
until  the  blue  colour  is  permanent.  The  determination  is 
repeated  with  a  second  portion  of  25  cubic  centimetres. 
This  method  is  based  upon  the  following  reaction  between 
the  arsenious  acid  solution  and  the  sulphuretted  hydrogen  : 


As203  +  sH2S     =     As2S3  +  3H2O. 

i  cubic  centimetre  of  deci-normal  arsenious  acid  solution  = 
0*0024  gram  sulphur.  With  proper  care  this  method  affords 
very  accurate  results. 

Determination  of  Nitrogen.  —  From  the  experiments  of 
Schafhautl,  Despretz,  Boussingault,  Fremy,  and  others,  it 
appears  that  nitrogen  is  an  invariable  constituent  of  cast  and 
wrought  iron,  and  steel  ;  some  of  these  authorities  are  of 
opinion  that  when  present  in  estimable  proportion  it  exerts 
a  marked  influence  upon  certain  of  the  physical  properties 
of  the  metal.  The  iron  is  said  to  be  rendered  white  and 
brittle  and  less  liable  to  change  in  the  air  or  water  (Des- 
pretz ;  Buff).  According  to  Frdmy  the  nitrogen  exists  in 
the  iron  in  two  conditions,  since  when  the  metal  is  dissolved 


242  Quantitative  Chemical  Analysis. 

in  hydrochloric  acid,  a  portion  is  converted  into  ammonia, 
whilst  another  portion  remains  in  combination  with  the  car- 
bonaceous residue.  On  the  other  hand,  Erdmann,  Stahl- 
schmidt,  Stuart,  and  Baker  assert  that  the  quantity  of  nitro- 
gen usually  present  in  iron  is  too  minute  to  have  the  least 
influence  upon  the  metal.  The  following  methods  of  deter- 
mining the  nitrogen  present  in  iron  are  due' to  Ullgren.* 

Determination  of  the  Nitrogen  which  forms  Ammonia  on 
dissolving  the  Iron  in  Hydrochloric  Acid. — About  2  grams  of 
the  finely-divided  iron  are  treated,  in  a  flask  provided  with  a 
bent  tube,  with  a  solution  of  10  grams  crystallised  copper 
sulphate,  and  6  grams  fused  sodium  chloride.  When  the  iron 
is  dissolved,  add  excess  of  boiled  milk  of  lime,  boil  for  some 
time,  and  determine  the  evolved  ammonia  as  in  No.  VI 1 1.  p.  94. 

The  quantity  of  the  ammonia  in  the  distillate,  if  very 
minute,  may  be  determined  by  Nessler's  method  (see  Water 
Analysis). 

Determination  of  the  Nitrogen  present  in  the  Carbonaceous 
Residue. — By  combustion  with  mercuric  sulphate,  and 
measurement  of  the  evolved  nitrogen.  The  apparatus  em- 
ployed for  the  purpose  is  seen  in  fig.  63.  A  is  a  piece  of 
combustion  tubing  about  30  centimetres  long  ;  it  is  filled  as 
far  as  b  with  1 2  grams  of  dry  magnesite  or  bicarbonate  of 
soda.  From  b  to  c  is  placed  the  mixture  of  about  cri  gram 
of  the  carbonaceous  residue  (dried  at  120-130°),  with  from  3 
to  4  grams  of  the  mercuric  sulphate.  The  mixture  is  made 
in  a  small  glazed  porcelain  or  agate  mortar,  which  is  rinsed 
with  a  fresh  portion  of  the  sulphate  after  the  introduction  of 
the  mixture.  From  c  to  d  is  a  layer  of  coarsely-powdered 
pumice,  previously  mixed  with  mercuric  sulphatef  and  a  little 
water,  and  dried  :  its  object  is  to  prevent  the  possible  evolu- 
tion of  carbon  monoxide.  The  remainder  of  the  tube  is 

*  Ann.  d.  Chem.  u.  Pharm.  cxxiv.  70,  cxxv.  40. 

f  Preparation  of  the  mercuric  sulphate.  This  salt  is  obtained  by 
gradually  adding  4  parts  of  mercury  to  5  parts  of  strong  boiling  sulphuric 
acid,  and  heating  the  mixture  until  sulphur  dioxide  ceases  to  be  evolved, 
and  the  whole  is  converted  into  a  dry  saline  mass. 


Wrought  and  Cast  Iron,  &c.  243 

filled  with  fragments  of  pumice  soaked  in  a  concentrated  solu- 
tion of  potassium  bichromate,  and  allowed  to  drain:  its  object 
is  to  absorb  the  sulphur  dioxide  which  is  disengaged.  The 
various  mixtures  are  separated  by  plugs  of  recently-ignited 


FIG.  63. 


d\ 


asbestos.  The  tube  is  fitted  with  a  caoutchouc  cork  and 
bent  tube.  The  vessel  B  is  designed  to  collect  and  measure 
the  evolved  nitrogen.  The  narrow  portion  holds  about  20 
cubic  centimetres,  and  is  graduated  into  y^ths  of  a  cubic 
centimetre  :  the  bulb  holds  about  40  cubic  centimetres, 
and  the  lower  portion  from  20  to  30  cubic  centimetres.  Fill 
the  tube  with  mercury  and  invert  it  in  the  trough.  By  means 
of  a  pipette  pass  up  potash  solution  (i  part  potassium  hydrate 
and  2  of  water)  until  the  bulb  is  nearly  filled  (to  within  10 
cubic  centimetres),  and  then  add  15  cubic  centimetres  of  a 
clear  and  saturated  solution  of  tannic  acid.  Now  gently 
heat  the  magnesite  or  bicarbonate  of  soda  at  the  extreme 
end  of  the  tube,  and  gradually  heat  the  tube  until  about 
half  the  substance  has  been  decomposed  :  the  air  within  the 
apparatus  is  thus  expelled.  Bring  the  end  of  the  delivery- 
tube  under  B,  heat  from  b  to  c  very  gently  to  remove  any 
moisture  present,  then  heat  from  c  to  d  gradually ;  and  when 


244  Quantitative  Chemical  Analysis. 

this  portion  of  the  tube  is  nearly  red  hot,  heat  the  part 
b  c  to  a  strong  red  heat.  Heat  from  b  to  d  until  no  more  gas  is 
evolved,  and  sweep  out  the  gases  within  the  tube  by  heating 
the  undecomposed  portion  of  magnesite.  Close  the  end  of 
B  with  the  thumb,  and  transfer  the  tube  to  a  vessel  of  water : 
the  mercury  and  potash  will  be  replaced  by  water ;  adjust 
the  levels  of  the  liquid  within  and  without  the  tube,  and  read 
off  the  volume  of  nitrogen,  making  the  necessary  corrections 
for  tension  of  aqueous  vapour,  temperature,  and  pressure, 
and  calculate  the  weight  of  the  nitrogen  found. 

Determination  of  the  Silicon,  Iron,  Manganese,  Cobalt, 
Nickel,  Zinc,  Alumina,  Titanic  Acid,  Alkaline  Earths, 
and  Alkalies* — Weigh  out  about  10  grams  of  the  finely- 
divided  iron  into  a  porcelain  basin,  cover  it  with  a  large 
watch-glass,  and  dissolve  it  in  moderately  dilute  hydrochloric 
acid,  add  a  few  drops  of  sulphuric  acid,  and  evaporate  to 
dryness  on  the  water.-bath,  heating  until  the  mass  no  longer 
smells  of  hydrochloric  acid.  Moisten  the  dried  salt  with 
hydrochloric  acid,  heat  on  the  water-bath,  add  water,  filter  into 
a  capacious  porcelain  basin,  wash  and  dry  the  residue.  Set  it 
aside  and  label  it  '  Pp.  I.'  Heat  the  filtrate  with  nitric  acid, 
boil,  add  water  until  the  liquid  measures  at  least  1,500  cubic 
centimetres,  and  gradually  add  ammonium  carbonate  in  dilute 
solution  until  the  fluid  just  loses  its  transparency  (it  must  not, 
however,  show  any  sign  of  distinct  precipitate).  Heat  to 
boiling,  and  maintain  the  liquid  in  ebullition  until  the  car- 
bonic acid  is  expelled.  If  the  solution  is  not  too  concen- 
trated the  ferric  hydrate  separates  rapidly.  Add  now  a  few 
drops  of  ammonia,  filter,  and  wash  the  precipitate  with  water 
containing  a  little  ammonium  chloride.  The  only  condition 
necessary  to  ensure  success  is  the  proper  dilution  of  the 
liquid  :  for  the  quantity  of  iron  taken  it  should  not  measure 
less  than  \\  litre.  Dry  the  washed  precipitate,  set  it  aside, 
and  label  it  <  Pp.  II.' 

*  Compare  Lippert,  Zeits.  f.  anal.  Chemie,  ii.  39. 


Wrought  and  Cast  Iron,  &c.  245 

Add  ammonia  in  slight  excess  to  the  nitrate  from  '  Pp.  II.,' 
and  boil  until  the  free  ammonia  is  nearly  expelled,  filter, 
and,  without  washing,  redissolve  in  hydrochloric  acid,  and 
again  precipitate  with  ammonia.  Filter,  wash  and  dry  the 
precipitate  :  call  it  'Pp.  III.' 

Mix  the  two  filtrates,  acidify  with  hydrochloric  acid,  con- 
centrate in  a  porcelain  basin,  transfer  to  a  small  flask,  add 
ammonia  and  ammonium  sulphide,  cork  the  flask,  and  let 
the  liquid  stand  in  a  warm  place  for  at  least  twenty- four  hours. 
Decant  the  clear  fluid  through  a  filter,  and  wash  it  with 
water  containing  a  little  ammonium  sulphide  ;  allow  it  to 
drain  as  far  as  possible,  spread  the  filter  on  a  glass  plate,  and 
wash  off  the  precipitate  into  a  small  flask  ;  add  acetic  acid, 
and  cork  the  flask.  Label  the  flask  '  Pp.  IV.' 

Transfer  the  filtrate  to  a  porcelain  basin,  evaporate,  add 
nitric  acid,  and  heat  until  the  ammonium  chloride  is  de- 
composed. Evaporate  to  dryness,  dissolve  in  a  little 
water,  filter  if  necessary,  add  ammonium  oxalate,  and,  after 
standing  twenty-four  hours,  filter  off  the  calcium  oxalate, 
wash  it  and  convert  it  into  lime  by  ignition.  Evaporate  the 
filtrate  to  dryness  in  a  platinum  basin,  ignite  at  first  gently 
and  then  to  a  red  heat.  Treat  with  water,  and  filter  off  the 
magnesia  and  weigh  it.  The  solution  of  the  alkalies  is  acidi- 
fied with  hydrochloric  acid,  evaporated  to  dryness,  the  mixed 
chlorides  weighed,  and  separated  as  in  No.  IV.  Part.  II. 

The  residue  marked  *  Pp.  I. '  contains  the  graphite,  silica, 
a  portion  of  the  phosphorus  as  phosphide  of  iron,  titanic 
acid,  barium  sulphate.  Fuse  it  with  sodium  carbonate 
and  a  little  nitre,  soften  the  mass  with  water,  add  hydro- 
chloric acid  in  excess,  evaporate  to  dryness  and  separate 
the  silica.  Weigh  it,  and  treat  it  in  a  platinum  basin 
with  a  moderately-concentrated  solution  of  sodium  car- 
bonate :  if  it  dissolves  completely,  it  is  pure  ;  if  a  weighable 
amount  remains,  determine  its  quantity  and  test  it  for 
titanic  acid  and  barium  sulphate.  To  the  filtrate  from  the 
silica,  add  ammonia,  filter  off  any  precipitate  which  forms, 


246  Quantitative  Chemical  Analysis. 

redissolve  it  in  hydrochloric  acid,  and  again  add  ammonia. 
Filter  and  dry,  and  add  the  precipitate  which  forms  to 
'  Pp.  III.'  To  the  nitrate  add  ammonium  sulphide,  and 
add  any  precipitate  which  separates  out  to  *  Pp.  IV.' 
Test  the  nitrate  for  the  alkaline  earths  in  the  manner 
above  directed,  and  if  their  amount  admits  of  determination, 
weigh  them. 

'Pp.  Il.'and  'Pp.  III.'  contain  the  ferric  oxide  and  alumina, 
and  the  small  quantity   of  titanic  acid  which   may  have 
passed  into  the  hydrochloric  acid  solution.    The  mixed  pre- 
cipitates are  placed  in  several  porcelain  or  platinum  boats, 
and   strongly  ignited  in  a  glass  tube  in  a  current  of  dry 
hydrogen   until  the  formation  of  water  ceases.     Allow  the 
reduced  metal  to  cool  in  the  current  of  the  gas,  and  treat 
the  mixture  with  dilute  nitric  acid  (i  part  of  acid  to  30  of 
water),   until  the  iron  is  dissolved  :  filter  the  liquid  into  a 
litre  flask,  dilute  to  1,000  cubic  centimetres,  and  determine 
the  iron  by  precipitating  an  aliquot  portion  by  means  of 
ammonia.     This  method  is  better  than  that  of  determining 
the  iron  in  a  fresh  portion  of  the  sample,  unless  the  sample 
is  perfectly  homogeneous.     The  residue  left  after  treatment 
with  dilute  nitric  acid  is  fused  with  acid- potassium  sulphate, 
digested  with  water,  and  any  residual  silica  filtered  off  and 
weighed.     The  filtrate  is  boiled  for  some  time,  and  if  any 
titanic  acid  separates  out,  it  is  filtered  off  and  weighed.  Add 
ammonia  and  boil :  dissolve  the  precipitate  in  hydrochloric 
acid,  transfer  the  liquid  to  a  small  test-tube,  add  a  little 
tartaric  acid,   then    ammonia,    and    ammonium    sulphide. 
Allow  the  liquid  to  stand  until  any  iron  sulphide  which  forms 
has  completely  settled,  filter,  redissolve  in  hydrochloric  acid, 
boil  with  a  few  drops  of  nitric  acid,  add  ammonia,  and  filter 
off  the  ferric  hydrate  ;  dry,  ignite,  and  weigh.    To  the  yellow 
filtrate  containing  the  alumina,  add  a  little  pure   sodium 
carbonate  and  nitre,  evaporate  to  dryness,  and  ignite  until 
the  mass  is  completely  white.     Rinse  the   residue   into   a 
beaker,   dissolve  in  hydrochloric  acid,  and  add  excess  of 
ammonia.    Filter  off  the  precipitate,  wash,  dry,  and  weigh  it. 


Wrought  and  Cast  Iron,  &c.  247 

Mix  the  filtrate  with  a  few  drops  of  magnesium  sulphate 
solution  :  if  a  precipitate  of  magnesium-ammonium  phosphate 
forms  after  standing,  calculate  the  weighed  precipitate  as 
A1PO4.  If  no  precipitate  is  formed,  the  amount,  of  phosphoric 
acid,  determined  as  under,  is  to  be  subtracted  from  the 
weighed  precipitate.  The  remainder  gives  the  amount  of 
alumina.  This  precipitate  will  also  contain  any  chromium 
present  in  the  iron.  Its  amount  is  in  general  exceedingly 
minute.  In  case  the  quantity  happens  to  be  more  consider- 
able than  usual,  fuse  the  mixed  oxides  with  a  little  sodium 
carbonate  and  nitre,  dissolve  in  water,  add  a  small  quantity 
of  ammonium  nitrate,  evaporate  nearly  to  dryness,  add  a  little 
water,  filter,  reduce  the  alkaline  chromate  with  sulphurous 
acid,  and  precipitate  the  chromic  oxide  with  ammonia,  boil 
for  some  time,  filter,  dry,  and  weigh.  The  difference 
between  the  weight  of  the  oxide  thus  obtained  and  that  of 
the  original  precipitate  gives  the  alumina. 

*  Pp.  IV.'  consists  principally  of  manganese  sulphide  : 
it  may  also  contain  zinc,  copper,  nickel,  and  cobalt.  By 
digestion  with  acetic  acid  the  greater  portion  of  the  man- 
ganese sulphide  will  have  been  dissolved  :  filter,  spread  the 
filter  containing  the  residue  on  a  glass  plate,  and  wash  the 
precipitate  into  a  small  beaker  containing  sulphuretted  hy- 
drogen water  acidulated  with  hydrochloric  acid.  Allow  the 
liquid  to  stand  for  a  short  time  :  the  zinc  and  the  remainder 
of  the  manganese  pass  into  solution;  the  copper  (which  is 
estimated  as  under),  cobalt,  and  nickel  remain  undissolved. 
Filter,  evaporate  the  acid  solution  to  a  few  cubic  centimetres, 
add  excess  of  soda  solution,  boil,  and  filter  off  any  man- 
ganese precipitated,  redissolve  it  in  hydrochloric  acid,  and 
add  the  liquid  to  the  acetic  acid  solution.  Pass  sulphuretted 
hydrogen  through  the  alkaline  filtrate  and  treat  the  zinc 
sulphide  as  in  No.  XIII.  Part  II.  p.  106, 

The  filter  containing  the  nickel,  cobalt,  and  copper  is  dried 
and  incinerated,  dissolved  in  a  few  drops  of  aqua  regia, 
treated  with  sulphuretted  hydrogen,  filtered  if  necessary, 
and  the  nickel  and  cobalt  separated  as  in  No.  XVI.  Part  IV. 


248  Quantitative  Chemical  Analysis. 

To  the  solution  containing  the  manganese,  add  sodium 
carbonate  in  slight  excess,  boil,  filter,  wash  the  manganous 
carbonate,  and  ignite  it  until  the  weight  is  constant.  It  con- 
sists of  mangano-manganic  oxide.  Care  must  be  taken  to 
remove  the  precipitate  as  completely  as  possible  from  the 
filter  before  it  is  incinerated. 

Determination  of  the  Phosphorus,  Copper,  Arsenic,  and 
Antimony. — Weigh  out  about  10  grams  of  the  iron  in  a  state 
of  fine  powder  into  a  capacious  flask,  place  a  small  funnel  in 
the  neck,  and  pour  strong  nitric  acid  in  small  quantities  at 
a  time  over  the  metal.  Warm  the  liquid,  and  when  all 
visible  action  is  at  an  end,  heat  the  residue  with  a  fresh  por- 
tion of  nitric  acid.  Mix  the  solutions,  add  hydrochloric  acid 
and  evaporate  to  complete  dryness :  again  add  hydrochloric 
acid  and  evaporate  a  second  time  to  dryness.  Dissolve  in 
water,  and  saturate  the  liquid  with  sulphuretted  hydrogen. 
After  the  precipitate  has  completely  settled,  filter  into  a 
large  flask,  dry  the  precipitate,  and  digest  it  with  carbon 
disulphide  to  remove  the  sulphur.  Separate  the  copper, 
arsenic,  and  antimony  in  the  black  residue  which  remains, 
according  to  No.  XVI.  Part  IV. 

Pass  a  current  of  carbonic  acid  through  the  filtrate  con- 
tained in  the  flask  to  dissipate  the  dissolved  sulphuretted 
hydrogen,  add  a  small  quantity  of  ferric  chloride  solution, 
nearly  neutralise  with  sodium  carbonate,  add  a  little  barium 
carbonate,  and  cork  the  flask  ;  after  standing  about  twenty- 
four  hours,  the  precipitate  will  contain  the  whole  of  the 
phosphoric  acid  ;  filter  it  off,  wash,  dissolve  in  hydrochloric 
acid,  remove  the  barium  by  the  addition  of  sulphuric  acid, 
filter,  concentrate  the  filtrate,  add  molybdic  acid  solution, 
and  proceed  as  in  No.  XIX.  Part  IV.  p.  218. 

The  residue  unattacked  by  aqua  regia  not  unfrequently 
contains  phosphide  of  iron.  Fuse  it,  therefore,  with  sodium 
carbonate  and  nitre,  extract  with  water,  and  test  the  solution 
for  phosphoric  acid. 


Iron  Slags. 


249 


Determination  of  Admixed  Sla%  in  Cast  Iron. 

By  treating  a  piece  of  the  metal  by  Weyl's  method  (p.  231), 
with  very  dilute  hydrochloric  acid,  any  admixed  slag  is  not  de- 
composed, and  remains  with  the  graphite,  &c.  The  residue  is 
collected  on  a  filter,  washed,  and  ignited  in  a  platinum  crucible 
until  the  carbon  is  consumed.  The  ignited  mass  is  boiled 
with  solution  of  sodium  carbonate  to  dissolve  the  free  silica, 
the  insoluble  matter  ignited  first  in  a  stream  of  hydrogen,  and 
then  in  dry  chlorine  (free  from  air),  and  treated  with  dilute 
hydrochloric  acid,  and  again  with  boiling  sodium  carbonate 
solution.  Filter  off  the  insoluble  matter,  dry  it,  and  weigh 
it  as  slag. 

The  following  analysis  by  Fresenius  of  '  Spiegel-eisen,' 
produced  from  the  spathic  ore  of  Stahlberg,  near  Miisen, 
was  made  by  the  foregoing  method  :  — 

.    0*014 

•     0-997 


Iron  . 

.  82-860 

"Magnesium 

.     0-045 

Nitrogen 

Manganese 

.  10-707 

Calcium    . 

.     0-091 

Silicon 

Nickel      . 

.     o'oi6 

Potassium 

.     0-063 

Carbon 

Cobalt      . 

trace 

Arsenic     . 

.     0-007 

Slag. 

Copper     . 

.     0-066 

Antimony 

.     0-004 

Aluminium 

.     0-077 

Phosphorus 

•     0-059 

Titanium  . 

.     o'oo6 

Sulphur    . 

.     0-014 

0-665 


100-014 


XXII.     IRON  SLAGS. 

The  slag  produced  in  the  manufacture  of  cast  iron  may  be 
regarded  as  a  double  silicate  of  lime  and  alumina,  in  which  a 
portion  of  the  lime  is  replaced  by  ferrous  and  manganous 
oxides,  magnesia,  and  alkalies.  Phosphoric  acid  and  calcium 
sulphide  are  also  usually  present.  The  average  composition 
of  the  blast-furnace  slag  is  represented  by  the  following 
analysis  :  — 


Silica  .  .  41*85 
Alumina  .  .  1473 
Ferrous  oxide  .  2-63 
Manganous  oxide  1-24 


Lime 

Magnesia . 
Potash  . 
Calcium  . 


30-99 
4-76 
1-90 


Sulphur   . 
Phosphoric  acid 


0-92 
0-15 


100-32 


Occasionally,  however,  either  from  carelessness  or  from 
defective  working  of  the  furnaces,  the  amount  of  iron  in  the 
slag  is  considerably  augmented.  Some  slags  are  completely 


2 5°  Quantitative  Chemical  Analysis. 

decomposed  by  treatment  with  hydrochloric  acid  :  others 
are  only  partially  acted  upon.  All  of  them  yield  more  or 
less  sulphuretted  hydrogen  on  heating  with  hydrochloric 
acid.  The  amount  thus  evolved  may  be  determined  by  one 
of  the  methods  described  on  p.  239. 

The  methods  of  analysis  described  in  Nos.  XI.  and  XII. 
Part  II.  are  generally  applicable  to  slags  which  resist  the 
action  of  hydrochloric  acid. 

A  very  convenient  method  of  decomposing  slags  is  to 
heat  them  in  a  finely-divided  state  in  a  sealed  tube  for  two 
hours  for  200°  in  an  air  bath,  with  a  mixture  of  three  parts 
by  weight  of  strong  sulphuric  acid  and  one  of  water,  or  with 
hydrochloric  acid  containing  25  per  cent.  HC1.  About  one 
gram  of  the  powder  is  introduced  into  a  strong  tube  of 
Bohemian  glass  rounded  at  one  end  ;  the  other  end  is  thick- 
ened before  the  blow-pipe  and  drawn  out.  Add  the  acid, 
and  carefully  seal  the  tube.  When  the  action  is  at  an  end, 
open  the  tube  after  cooling,  and  rinse  out  its  contents  into  a 
porcelain  dish,  and  evaporate  to  dryness  in  the  ordinary 
way  to  render  the  silica  insoluble.  The  remainder  of  the 
analysis  is  conducted  after  Nos.  XI.  and  XII.  Part  II.  This 
process  is  often  applicable  to  the  analysis  of  natural  silicates : 
the  determination  of  mixtures  of  ferric  and  ferrous  oxides 
may  also  be  conveniently  made  by  this  method. 

Tap-cinder  may  be  analysed  by  the  above  method,  or 
according  to  the  processes  given  in  Nos.  XX.  XXI.  Part  IV. 

XXIII.     ASSAY  OF  ZINC-ORES. 

From  0-5  gram  to  i  gram  (according  to  its  supposed  rich- 
ness) of  the  finely-powdered  ore  is  dissolved  in  aqua  regia, 
the  solution  is  evaporated  to  dryness,  the  residue  heated 
with  water,  filtered,  and  mixed  with  ammonium  carbonate 
and  ammonia  solutions.  The  liquid  is  gently  heated  on  a 
sand-bath  for  half  an  hour,  filtered,  and  the  insoluble  matter 
washed  with  ammonium  acetate  solution.  The  filtrate  con- 
taining the  zinc  is  mixed  with  an  excess  of  sodium  or  am- 
monium sulphide,  and  after  standing  for  a  short  time  the 


Zinc  Ores.  251 

cipitate  is  washed  first  by  decantation  and  afterwards  on  the 
filter  with  warm  water  containing  ammonia,  until  the  filtrate 
no  longer  discolours  an  alkaline  solution  of  lead  acetate. 
The  filter  is  pierced,  and  the  zinc  sulphide  is  carefully 
washed  down  into  a  500  c.c.  flask  containing  an  excess  of 
ferric  chloride  solution  and  some  free  hydrochloric  acid. 
The  flask  is  well  closed  and  set  aside  in  a  warm  place  for  a 
short  time.  The  mixture  should  occasionally  be  agitated  to 
accelerate  the  reaction.  The  zinc  sulphide  in  contact  with 
the  ferric  chloride  and  free  hydrochloric  acid  is  converted 
into  zinc  chloride,  sulphur  is  separated,  and  the  iron  is  re- 
duced to  the  state  of  ferrous  chloride. 

ZnS   +   Fe2Cl6   =    ZnCl2   +    2FeCl2  +   S. 

The  solution  (which  should  have  a  yellow  colour,  indica- 
ting an  excess  of  ferric  chloride,  and  be  free  from  any  smell 
of  sulphuretted  hydrogen)  is  diluted  to  the  containing-mark, 
well  shaken,  and  an  aliquot  portion  of  the  liquid  titrated 
with  acid-chromate  of  potassium  solution.  If  the  liquid  is 
quite  cold  and  dilute,  the  free  sulphur  exercises  no  influence 
upon  the  result. 

The  alkaline  solution  of  zinc  may  also  be  titrated  with  a 
solution  of  sodium  sulphide  of  known  strength.  This  method 
is  largely  used  in  many  zinc  works  on  the  Continent. 
Saturate  a  solution  of  soda  with  sulphuretted  hydrogen,  and 
add  a  second  quantity  of  the  alkaline  liquid  until  the  smell 
of  the  gas  is  no  longer  perceptible.  Dissolve  10  grams  of 
pure  zinc  in  dilute  sulphuric  acid  (taking  care  not  to  use  a 
very  great  excess  of  acid),  and  dilute  the  solution  to  i  litre. 
Also  dissolve  a  few  crystals  of  sodium  tartrate  in  water,  add 
a  small  quantity  of  caustic  soda  and  lead  acetate,  and  heat 
the  mixture  until  the  liquid  is  clear. 

Transfer  25  c.c.  of  the  standard  zinc  solution  to  a  beaker ; 
add  a  mixture  of  ammonium  carbonate  and  ammonia  suffi- 
cient to  redissolve  the  precipitate  first  formed,  and  spread  a 
few  drops  of  the  alkaline  lead  solution  on  a  piece  of  filter- 
paper  placed  on  a  porcelain  slab  or  plate,  and  run  in  the 
solution  of  sodium  sulphide  until  a  drop  of  the  liquid  with- 


252  Quantitative  Chemical  Analysis. 

drawn  by  a  glass  rod  and  brought  into  contact  with  the  lead 
acetate  forms  a  black  ring  at  the  point  of  contact.  If  neces- 
sary, the  solution  of  the  sodium  sulphide  is  diluted  to  a  con- 
venient strength  for  titration,  and  its  exact  value  again  de- 
termined in  the  above  manner  on  quantities  of  25  c.c.  or  50 
c.c.  of  the  standard  zinc  solution. 

The  amount  of  zinc  in  the  ammoniacal  liquid  obtained 
by  treating  the  ore  is  then  determined  in  exactly  the  same 
manner  by  the  sodium  sulphide  solution.  The  strength  of 
the  solution  of  sodium  sulphide  must  be  re-determined  before 
each  series  of  experiments,  as  it  experiences  alteration  by 
exposure  to  the  air. 

Many  zinc-blendes  contain  notable  quantities  of  copper, 
which  by  combining  with  the  sodium  sulphide  increases  the 
amount  of  zinc  apparently  present.  The  safest  plan  in  such 
a  case  is  to  remove  the  copper  by  sulphuretted  hydrogen  in 
an  acid  solution ;  filter,  evaporate  the  filtrate  with  nitric  acid, 
dilute,  add  ammonia,  and  proceed  as  directed. 

In  the  case  of  ores  which  contain  alumina  and  manganese, 
zinc  is  more  accurately  estimated  by  a  standard  solution  of 
potassium  ferrocyanide.  The  solution  of  the  ore  prepared 
as  above  is  acidified  with  hydrochloric  acid,  and  the  ferro- 
cyanide solution  (previously  standardised  by  means  of  pure 
zinc)  is  added  until  a  drop  withdrawn  from  the  liquid  gives  a 
brown  colour  with  solution  of  uranium  nitrate  placed  on  a 
porcelain  slab.  (Comp.  pp.  333,  334.) 

XXIV.     ASSAY  OF  TIN-ORES. 

The  amount  of  tin  in  its  ores  may  be  easily  estimated  by 
fusing  with  potassium  cyanide :  the  process,  although  not 
absolutely  correct — especially  in  presence  of  lead  or  copper 
• — yields  results  of  sufficient  accuracy  for  technical  purposes. 
About  6  or  8  grams  of  the  finely-powdered  ore  is  placed  in 
a  smooth  porcelain  mortar,  and  intimately  mixed  with  five 
times  its  weight  of  commercial  potassium  cyanide.  The 
mixture  is  projected  into  a  small  clay  crucible,  in  the  bottom 
of.  which  a  quantity  of  powdered  potassium  cyanide  has  been 


Tin-Ores.  253 

previously  placed,  sufficient  to  form  a  layer  of  i  or  2  centi- 
metres in  depth,  and  the  mortar  is  rinsed  with  a  fresh  quantity 
of  cyanide,  which  is  poured  on  the  top  of  the  mixture.  It 
is  advisable  by  way  of  control  to  prepare  two  such  crucibles, 
and  to  take  the  higher  result  as  the  true  one.  The  crucibles 
are  heated  to  a  moderate  red  heat,  and  the  cyanide  is  kept  in 
fusion  for  10  or  15  minutes  :  they  are  removed  from  the  fire 
and  gently  tapped,  to  promote  the  formation  of  a  single 
button  of  the  reduced  metal.  After  cooling  they  are  broken, 
and  the  buttons  of  metal  are  extracted  and  weighed  after 
the  adhering  flux  has  been  removed.  The  saline  mass 
should  be  triturated  with  water  in  order  to  be  certain  that 
the  reduction  has  been  effectual  and  that  the  whole  of  the 
metal  has  been  collected  in  one  piece.  The  silica  in  the  ore 
unites  with  the  alkaline  carbonate  always  contained  in  the 
commercial  cyanide.  If  the  ore  contains  any  considerable 
portion  of  lead  or  copper,  these  must  first  be  removed  by 
digestion  with  strong  hydrochloric  acid. 

XXV.  SEPARATION  OF  TIN  FROM  TUNGSTEN. 
Commercial  stannates  of  soda  are  frequently  mixed  with 
sodium  tungstates.  The  best  method  of  determining  the 
proportion  of  the  two  acids  is  to  fuse  the  mixture  with 
potassium  cyanide,  when  the  tin  only  is  reduced  to  the 
metallic  state.  About  2  grams  of  the  powdered  salt  is 
mixed  with  four  times  its  weight  of  fused  and  powdered 
potassium  cyanide,  and  the  mixture  heated  in  a  porcelain 
crucible.  By  treating  the  fused  mass  with  water  the  alkaline 
tungstate  dissolves,  together  with  the  excess  of  the  cyanide : 
the  reduced  metal  is  washed,  and  converted  into  oxide  by 
treatment  with  nitric  acid.  The  filtrate  is  heated  with  nitric 
acid  to  decompose  the  potassium  cyanide,  evaporated  nearly 
to  dryness,  the  residue  dissolved  in  alkali,  acidified  with 
nitric  acid,  and  the  tungstic  oxide  precipitated  by  means  of 
mercurous  nitrate.  The  mercurous  tungstate  is  washed  with 
water  containing  a  little  mercurous  nitrate.,  dried,  and  con- 
verted into  tungstic  oxide  by  ignition  in  contact  with  air. 


254  Quantitative  Chemical  Analysis. 

XXVI.     WOLFRAM. 

This  mineral  is  a  tungstate  of  iron  and  manganese 
(FeMn")  WO4,  in  which  the  proportion  of  the  two  metals  is 
variable.  It  occurs  associated  with  tin -ore,  tungstate  of  cal- 
cium, galena,  &c.,  in  Cornwall,  Cumberland,  in  the  Hebrides, 
France,  Bohemia,  and  North  America. 

The  finely-divided  mineral  is  heated  with  aqua  regia  until 
it  is  completely  decomposed.  The  solution  is  evaporated 
to  complete  dryness  over  the  water-bath,  water  added,  and 
the  manganese  and  ferric  chlorides  filtered  off.  The  residual 
tungstic  acid  is  washed  with  alcohol,  dissolved  in  ammonia, 
the  solution  filtered  from  any  residue,  evaporated  to  dryness 
in  a  capacious  porcelain  crucible,  gently  heated  to  expel 
ammonia,  and  ignited  in  contact  with  air.  The  tungstic 
oxide  is  then  weighed  :  it  should  have  a  pure  yellow  colour, 
free  from  any  greenish  tint. 

Test  the  residue  for  niobic  acid""  by  heating  it  in  the 
Bunsen  flame  with  microcosmic  salt :  the  oxide  of  niobium 
forms  a  colourless  bead  in  the  outer  flame,  but  a  violet- 
coloured  bead  inclining  to  blue  in  the  inner  flame  if  the  fused 
salt  is  saturated  with  the  oxide.  By  adding  a  trace  of  ferrous 
sulphate  the  colour  is  changed  to  blood-red.  As  very  similar 
reactions  are  afforded  by  tungstic  oxide,  care  must  be  taken 
to  ensure  that  the  whole  of  this  substance  is  removed  by 
ammonia  before  the  test  is  made.  Another  portion  may  be 
heated  with  a  bead  of  sodium  carbonate,  by  which  it  is  dis- 
solved, forming  whilst  hot  a  transparent  mass,  which  becomes 
turbid  on  cooling ;  if  whilst  still  hot  it  is  moistened  with  a 
drop  of  tin  chloride,  and  heated  in  the  lower  reducing  flame, 
it  gives  a  grey  mass,  which  dissolves  in  hydrochloric  acid, 
producing  a  light  amethystine  tint. 

The  alcoholic  filtrate  is  evaporated  to  dryness,  the  residue 
dissolved  in  water,  and  the  iron  and  manganese  separated 
as  in  No.  XIX.  Part  IV.  p.  220. 

*  Columbite,  a  niobate  of  iron  and  manganese,  not  unfrequently  oc- 
curs associated  with  wolfram- 


Scheelite :  Galena.  255 

XXVIL     SCHEELITE  (CaWO4). 

This  mineral  is  readily  decomposed  by  strong  nitric  acid. 
The  solution  is  evaporated  nearly  to  dryness,  alcohol  added, 
and  the  liquid  filtered.  Tungstic  oxide  is  left  undissolved  : 
calcium  nitrate  goes  into  solution.  The  oxide  is  treated  in 
the  manner  above  described  :  the  solution  is  evaporated 
nearly  to  dryness,  water  added,  and  the  lime  estimated  in 
the  usual  manner. 

XXVIII.     GALENA. 

This  substance  is  essentially  lead  sulphide,  but  it  is  almost 
invariably  mixed  with  more  or  less  iron,  copper,  silver,  anti- 
mony, and  zinc,  and  silicious  matter  (gangue). 

Determination  of  the  Sulphur. — The  ore  is  reduced  to  the 
finest  powder  and  dried  at  100°.  About  i  gram  of  the  sub- 
stance is  weighed  out  into  a  large  porcelain  crucible,  gently 
heated  with  potash  solution  (free  from  sulphate)  for  an  hour, 
and  a  slow  current  of  chlorine  conducted  into  the  liquid. 
The  galena  by  this  treatment  is  decomposed  ;  the  sulphur  is 
oxidised  to  sulphuric  acid,  which  combines  with  the  potash, 
and  the  lead  is  converted  into  binoxide.  The  liquid  is  fil- 
tered, acidified  with  hydrochloric  acid,  and  the  sulphur 
precipitated  as  barium  sulphate. 

Determination  of  the  Lead,  Iron,  Zinc,  &>c. — About  1*5  to 
2  grams  of  the  ore  are  oxidised  with  red  fuming  nitric  acid 
(B.P.  86°)  in  a  flask,  in  the  mouth  of  which  is  placed 
a  small  funnel.  The  sulphur  is  thus  completely  oxidised,  and 
the  galena  converted  into  lead  sulphate.  Evaporate  off  the 
excess  of  acid,  and  add  about  5  cubic  centimetres  of 
moderately-strong  sulphuric  acid,  and  evaporate  nearly  to 
dryness.  Add  about  20  cubic  centimetres  of  water,  filter, 
wash  the  residue  with  water  containing  sulphuric  acid,  and 
remove  the  sulphuric  acid  by  washing  with  alcohol,  otherwise 
the  paper  will  fall  to  pieces  on  being  dried.  Do  not  mix 
the  alcoholic  washings  with  the  acid  filtrate.  The  operation 


2 56  Quantitative  Chemical  Analysis. 

of  washing  must  be  done  without  delay,  and  with  as  little 
water  as  possible,  otherwise  a  perceptible  amount  of  lead  sul- 
phate will  be  dissolved.  The  residue  in  the  filter  is  dried 
and  ignited  in  a  weighed  porcelain  crucible  ;  care  must  be 
taken  to  remove  as  much  of  the  lead  sulphate  as  possible 
from  the  paper  before  incineration.  The  ash  may  be 
moistened  with  one  drop  of  nitric  acid  and  then  one  drop  of 
sulphuric  acid,  and  the  whole  carefully  dried,  ignited,  and 
weighed.  The  substance  consists  of  lead  sulphate  mixed 
with  sand,  silica  (gangue).  When  weighed,  it  is  carefully 
transferred  to  a  small  beaker  and  heated  with  hydrochloric 
acid,  which  dissolves  the  lead  sulphate,  leaving  the  silicious 
matter  unchanged.  Allow  the  liquid  to  become  clear  by 
standing,  and  pour  it  through  a  small  filter,  again  digest  with 
hydrochloric  acid  and  again  filter,  repeating  this  treatment 
three  or  four  times  until  the  filtrate  is  no  longer  blackened 
by  sulphuretted  hydrogen  water.  Wash  the  residue  on  to 
the  filter  with  hot  water,  dry  and  weigh,  and  subtract  the 
weight,  minus  the  filter  ash,  from  the  original  weighing  :  the 
difference  gives  the  amount  of  lead  sulphate. 

Pass  sulphuretted  hydrogen  through  the  filtrate,  and  deter- 
mine the  copper  and  antimony  as  in  No.  XVI.  Part  IV. 
Iron  and  zinc  are  precipitated  from  the  filtrate,  after  treat- 
ment with  sulphuretted  hydrogen,  by  means  of  ammonium 
sulphide  :  they  may  be  separated  as  in  No.  XVI.  Part  IV. 

Determination  of  Silver  in  Galena. — Galena  rarely  con- 
tains as  much  as  0*5  per  cent,  of  silver,  but  this  metal  can  be 
profitably  extracted,  even  when  it  does  not  exceed  one- 
twentieth  of  this  amount  in  the  ore.  The  exact  determina- 
tion of  the  small  quantities  of  silver  almost  universally  present 
in  galena  becomes,  therefore,  a  matter  of  importance.  When 
an  argentiferous  galena  is  smelted,  the  whole  of  the  silver  is 
found  in  the  reduced  metal.  About  50  or  60  grams  of  the 
finely-divided  galena  are  mixed  with  twice  their  weight  of 
sodium  carbonate  and  20  grams  of  nitre,  and  placed  in  a  clay 


Silver  in  Galena.  257 

crucible.  A  layer  of  well-dried  common  salt  (about  8  or  10 
millimetres  deep)  is  placed  over  the  mixture,  and  the  crucible 
is  heated  to  bright  redness.  It  is  allowed  to  cool,  broken, 
and  the  button  of  reduced  lead  extracted.  This  is  flattened 
on  an  anvil  and  freed  from  slag,  &c.,  by  rubbing  and  washing 
with  water.  The  following  equation  represents  the  reaction : 

4Na2CO3+  yPbS  =  4?b  +  3(PbSNa2S)  +  Na2SO4  +  4CO2. 

The  object  of  the  nitre  is  to  decompose  the  double  sulphide 
of  lead  and  sodium  :  the  lead  is  separated,  and  the  sodium 
sulphide  oxidised  to  sulphate.  The  button  of  lead  (which 
should  weigh  from  35  to  40  grams,  if  the  operation  has  been 
properly  conducted)  is  slowly  dissolved  in  pure  dilute  nitric 
acid  until  about  5  or  10  grams  of  the  metal  remain  :  this  is 
withdrawn  from  the  solution.  It  contains  the  whole  of  the 
silver.  It  is  dissolved  in  dilute  nitric  acid,  and  the  solution 
is  diluted  with  a  large  quantity  of  water.  A  few  drops  of 
highly-dilute  hydrochloric  acid  are  added,  and  the  liquid  is 
allowed  to  stand  until  the  silver  chloride  has  completely 
settled :  this  is  filtered  off,  washed  repeatedly  with  hot 
water,  and  weighed.  (Compare  also  No.  XXXIV.  p.  279. 

The  object  of  only  partially  dissolving  the  button  is  to 
avoid  the  presence  of  an  undue  amount  of  lead  in  solution, 
since  the  nitrate  of  this  metal  dissolves  silver  chloride  to  a 
perceptible  extent.  The  results  obtained  by  this  method,  if 
properly  conducted,  are  more  exact  than  those  given  by 
cupellation. 

A  very  ready  method  of  assaying  galena  for  technical  pur- 
poses consists  in  decomposing  the  ore  by  means  of  zinc  and 
hydrochloric  acid.  About  2  grams  of  the  finely-powdered 
sulphide  are  weighed  out  into  a  tall  beaker  and  covered  with 
a  piece  of  pure  zinc  about  an  inch  in  diameter  and  a  quarter 
of  an  inch  thick.*  Pour  into  the  beaker  about  120  cubic 
centimetres  of  dilute  hydrochloric  acid  ( i  part  acid  to  4  of 

*  Obtained  by  dropping  the  molten  metal  upon  a  smooth  surface  of 
wood  or  metal. 

S 


258  Quantitative  Chemical  Analysis. 

water),  cover  the  beaker  with  a  watch-glass,  and  gently  heat 
(to  40°  or  50°)  for  15  or  20  minutes,  occasionally  stirring 
the  liquid.  When  the  evolution  of  sulphuretted  hydrogen 
ceases,  and  the  liquid  becomes  clear,  the  decomposition  is 
complete.  The  supernatant  liquid  is  poured  on  to  a  filter, 
in  which  a  small  piece  of  zinc  is  placed,  and  the  zinc  and 
lead  in  the  beaker  washed  with  hot  water  by  decantation 
until  the  filtrate  has  no  longer  an  acid  reaction.  The  lead 
is  transferred  to  a  weighed  porcelain  crucible,  any  portions 
adhering  to  the  zinc  being  rubbed  off  by  a  glass  rod.  The 
small  particles  on  the  filter  are  washed  into  a  porcelain  basin 
and  added  to  the  crucible.  The  water  in  the  crucible  is 
poured  away  and  the  lead  gently  dried  in  a  current  of  coal 
gas  to  prevent  it  from  oxidising.  If  gangue  is  mixed  with 
the  reduced  lead,  its  amount  may  be  determined  by  dis- 
solving the  metal  in  dilute  nitric  acid,  and  washing,  drying, 
and  weighing  the  insoluble  residue. 

XXIX.     REFINED  LEAD. 

Recent  improvements  in  refining,  and  the  introduction 
of  such  improved  methods  of  desilverisation  as  Pattinson's 
crystallisation  process  or  Parkes'  zinc  process,  have  so  far 
perfected  the  process  of  manufacturing  softened  lead  that 
this  article  seldom  contains  less  than  99-9  per  cent,  of  the  pure 
metal.  Pure  as  such  lead  may  appear,  the  presence  in  it  of 
minute  traces  of  iron,  copper,  &c.,  yet  exercises  a  very  im- 
portant influence  in  its  application  to  the  manufacture  of 
vitriol  chambers,  evaporating  pans,  and  to  the  preparation  of 
white  lead,  flint  glass,  &c. 

The  following  substances  are  found  in  refined  lead  :  silver, 
copper,  bismuth,  cadmium,  zinc,  iron,  nickel,  and  antimony. 
Cobalt,  arsenic,  and  manganese  are  seldom  present  in  esti- 
mable quantity.  These  metals  are  derived  partly  from  the 
ores  and  partly  from  the  employment  of  Parkes'  process,  the 
impurities  being  introduced  in  the  zinc  used. 


Refined  Lead.  259 

The  Method  of  Analysis* — The  lead  to  be  analysed  is 
cut  up  into  large  pieces,  and  the  surface  of  each  piece  is 
scraped  with  a  clean  bright  knife  until  it  is  perfectly  bright 
and  free  from  any  apparent  impurity.  Weigh  off  two  por- 
tions of  200  grams  each  of  the  lead  into  flasks  of  1,500 
cubic  centimetres  capacity,  and  add  to  one  portion  about  500 
cubic  centimetres  of  pure  nitric  acid  of  sp.  gr.  i  -2,  and  so 
much  water  that  no  lead  nitrate  separates  out.  The  action 
of  the  acid  may  be  promoted  by  a  gentle  heat ;  care 
must  be  taken  not  to  employ  a  greater  excess  of  nitric  acid 
than  that  given.  The  solution  is  allowed  to  stand  from  12  to 
24  hours.  Since  200  grams  lead  give  310  grams  of  nitrate, 
and  i  part  of  lead  nitrate  requires  about  2  parts  of  water 
for  solution,  there  is  no  possibility  of  lead  nitrate  separating 
out  from  the  solution  if  it  measures  about  i  litre.  If  a  crys- 
tallisation occurs,  it  is  a  sign  that  too  great  an  amount  of 
nitric  acid  has  been  used,  lead  nitrate  being  far  more  in- 
soluble in  dilute  nitric  acid  than  in  water. 

To  the  other  200  grams  add  nitric  acid  of  the  above 
strength  in  small  portions  at  a  time,  always  keeping  the 
metal  in  excess,  and  heat  the  liquid  until  only  about  5  or  10 
grams  of  lead  remain  undissolved,  and  the  solution  com- 
mences to  turn  yellow  in  consequence  of  the  formation  of 
lead  nitrite.  In  the  residual  metal  the  whole  of  the  silver 
is  concentrated.  It  is  withdrawn  from  the  liquid,  dissolved 
in  nitric  acid,  the  solution  diluted  with  a  large  quantity  of 
water,  and  a  few  drops  of  a  solution  of  lead  chloride  added, 
or  i  cubic  centimetre  of  hydrochloric  acid  of  1*12  sp.  gr., 
previously  diluted  with  50  cubic  centimetres  of  water.  This 
quantity  of  acid  is  more  than  sufficient  to  precipitate  all  the 
silver  without  throwing  any  lead  chloride  out  of  solution. 
Set  the  beaker  aside  for  two  or  three  days.  Draw  off  the 
clear  liquid  by  means  of  a  syphon,  and  collect  the  precipitate 
on  a  small  filter,  wash  thoroughly  with  boiling  water,  dry  it, 

*  Fresenius,  Zeits.  fur  anal.  Chem.  vol.  viii.     1869. 
S  2 


260  Quantitative  Chemical  Analysis. 

and  incinerate  in  a  small  weighed  porcelain  crucible.  If  the 
amount  of  silver  chloride  is  so  considerable  that  there  is  a 
possibility  of  its  being  incompletely  reduced  by  the  combus- 
tion of  the  filter-paper,  the  residue  must  be  heated  for  a  few 
minutes  in  a  stream  of  hydrogen  before  weighing.  The 
amount  left,  after  subtracting  the  filter  ash,  gives  the  quan- 
tity of  silver  in  the  200  grams  of  lead.  The  refined  metal 
seldom  contains  more  than  0-0015  per  cent,  of  silver. 
(Mean  of  12  specimens,  0-0013  Per  cent) 

The  solution  of  the  other  portion  of  200  grams  is  used  for  the 
estimation  of  the  remaining  impurities.  As  a  rule  it  remains 
perfectly  clear  even  after  standing.  Occasionally,  however,  in 
the  case  of  lead  rich  in  antimony,  a  more  or  less  considerable 
precipitate  forms  after  a  time.  The  precipitate  is  filtered  off 
and  set  aside  ;  the  mode  of  examining  it  will  be  given  here- 
after. Bring  the  clear  solution  or  filtrate  into  a  2-litre  flask, 
add  115  grams  (about  62  or  63  cubic  centimetres)  pure  and 
concentrated  sulphuric  acid,  shake,  allow  to  cool,  and  fill  up 
to  the  mark ;  again  shake  the  liquid  so  as  to  mix  it  thoroughly, 
and  allow  the  precipitate  to  settle.  The  amount  of  sulphuric 
acid  to  be  added  is  so  calculated  that  about  10  or  12  grams 
are  in  excess.  As  soon  as  the  lead  sulphate  has  completely 
settled,  the  clear  liquid  is  drawn  off  by  means  of  a  syphon 
previously  filled  with  the  liquid.  In  this  way  rather  more 
than  1,750  cubic  centimetres  may  be  drawn  off.  Accurately 
measure  off  1,750  cubic  centimetres  of  the  solution,  and 
evaporate  it  in  a  porcelain  basin,  in  a  draught-place  free 
from  dust,  until  sulphuric  acid  fumes  make  their  appearance, 
indicating  that  the  nitric  acid  has  been  expelled.  Allow  the 
liquid  to  cool,  add  about  60  cubic  centimetres  of  water,  and 
filter  off  the  small  quantity  of  lead  sulphate  which  separates 
out,  and  wash  the  precipitate  with  a  little  water. 

The  slight  precipitate  of  lead  sulphate  frequently  retains 
small  quantities  of  antimony.  It  is  therefore  dissolved  in 
hydrochloric  acid,  and  the  solution  diluted  with  sulphuretted 
hydrogen  water,  the  liquid  warmed,  and  sulphuretted  hydro- 


Refined  Lead.  261 

gen  passed  through  it.  The  precipitate  is  allowed  to  sub- 
side, filtered,  washed,  the  filter-paper  spread  out  in  a 
porcelain  basin,  and  heated  with  a  solution  of  pure  am- 
monium or  potassium  sulphide,  to  which  a  small  quantity  of 
pure  sulphur  has  been  added.  Filter  the  solution,  wash, 
acidify  the  filtrate  with  hydrochloric  acid,  and  allow  the 
precipitate  to  settle  at  a  gentle  heat. 

The  solution  filtered  from  the  lead  sulphate,  and  diluted 
to  200  cubic  centimetres,  is  heated  to  about  70°,  and  treated 
with  sulphuretted  hydrogen,  allowed  to  stand  12  hours  at 
a  gentle  heat,  and  filtered  through  a  small  filter.  The 
washed  precipitate  is  heated  with  potassium  sulphide  solu- 
tion (containing  sulphur)  in  the  manner  above  described. 
The  filtrate  is  acidified  with  hydrochloric  acid  and  allowed 
to  stand  until  the  precipitate  has  completely  subsided. 

The  filter  and  residue  insoluble  in  potassium  sulphide  are 
heated  in  the  dish  nearly  to  boiling  with  dilute  nitric  acid 
(i  part  acid  1*2  sp.  gr.  and  2  of  water).     When  the  pre- 
cipitate is  dissolved,  filter,  wash  the  paper  slightly,  dry,  and 
incinerate,  and  add  the  ashes  to  the  nitric  acid  solution ; 
add  2  cubic  centimetres  of  dilute  sulphuric  acid,  and  evapo- 
rate until  the  nitric  acid  is  expelled  j   dilute  with  a  little 
water,  and  filter  off  the  small  quantity  of  lead  sulphate  which 
separates  out.    Nearly  neutralise  the  filtrate  with  pure  caustic 
potash,   add   sodium   carbonate,   and  a   small   quantity  of 
potassium  cyanide  solution  free  from  potassium  sulphide, 
and  heat  gently.     If  a  precipitate  is  formed,  it  is  filtered  off, 
washed,  and  dissolved  in  dilute  nitric  acid,  and  the  bismuth 
precipitated  by  ammonium  carbonate,  and  weighed  as  oxide. 
To   the  filtrate  a  little  more  potassium  cyanide  is  added, 
together  with   a   few  drops  of  potassium    sulphide.     The 
precipitate  which  ensues   contains  the  cadmium  and  silver. 
It  is  filtered  off,  dissolved  in  dilute  nitric  acid,  the  silver 
precipitated  by  hydrochloric  acid,  the   filtrate   evaporated 
nearly   to  dryness,  and  a  few  drops  of  sodium  carbonate 
solution  added.     The  cadmium  precipitate  is  filtered  off, 


262  Quantitative  Chemical  Analysis. 

dried,  ignited,  and  weighed  as  oxide.  The  reduction  and 
volatilisation  of  the  cadmium  may  be  prevented  by  moisten- 
ing the  filter  with  ammonium  nitrate.  The  nitrate  from  the 
mixed  silver  and  cadmium  sulphides  is  mixed  with  a  small 
quantity  of  sulphuric  and  nitric  acids,  and  evaporated  nearly 
to  dryness  ;  a  few  drops  of  hydrochloric  acid  are  added,  and 
the  solution  heated  until  the  last  traces  of  hydrocyanic  acid 
have  disappeared ;  the  solution  is  filtered,  if  necessary,  and 
the  copper  precipitated  and  weighed  as  sulphide. 

When  cadmium  is  absent,  the  separation  of  the  copper  and 
bismuth  may  be  effected  by  means  of  ammonia  and  am- 
monium carbonate.  In  this  case  it  must  not  be  forgotten 
that  the  silver  is  to  be  removed  by  the  addition  of  hydro- 
chloric acid  before  the  copper  is  precipitated  as  sulphide. 

The  precipitates  obtained  by  acidifying  the  sulphide  of 
ammonium  solution  are  filtered  off,  washed,  dried,  and 
repeatedly  treated  with  carbon  disulphide  to  remove  the 
sulphur.  The  little  filter  and  the  residue  are  then  together 
warmed  with  a  few  drops  of  red  fuming  nitric  acid,  the 
porcelain  basin  being  covered  with  a  watch-glass  j  the  solu- 
tion is  heated  to  expel  the  excess  of  nitric  acid,  sodium  car- 
bonate added  in  slight  excess,  and  then  a  small  quantity  of 
sodium  nitrate.  The  solution  is  evaporated  to  dryness,  and 
carefully  heated  until  the  residue  melts  and  the  mass  be- 
comes white.  When  cold  the  fused  mass  is  transferred  to 
a  small  mortar  and  carefully  broken  up  by  rubbing  it  in  a 
little  cold  water.  The  solution  is  filtered  and  the  residue  is 
washed  with  water  containing  alcohol.  The  sodium  anti- 
moniate  is  dissolved  in  hydrochloric  acid,  to  which  a  little 
tartaric  acid  has  been  added,  and  the  solution  treated  with 
sulphuretted  hydrogen,  and  set  aside  for  a  few  hours. 

The  soluble  portion  of  the  fused  mass  which  contains  all 
the  arsenic,  together  with  a  little  antimony,  is  evaporated  to 
expel  the  alcohol,  an  excess  of  sulphuric  acid  is  added,  and 
the  solution  again  evaporated  to  expel  the  nitric  acid,  water 
added,  the  liquid  heated  to  70°  C,  and  treated  with  sulphu- 


Refined  Lead.  263 

retted  hydrogen.  When  the  precipitate  has  settled,  it  is 
filtered  through  a  small  filter  and  washed  with  water.  It  is 
then  treated  on  the  filter  with  a  cold  concentrated  solution 
of  ammonium  carbonate,  the  filtrate  being  repeatedly  poured 
back  on  to  the  filter,  to  obviate  the  use  of  a  large  excess  of 
ammonium  carbonate.  The  arsenic  sulphide  is  dissolved : 
the  antimony  sulphide  mixed  with  a  little  sulphur  remains  un- 
dissolved.  This  residue  is  dissolved  in  a  little  strong  hydro- 
chloric acid  diluted,  and  treated  with  sulphuretted  hydrogen, 
and  the  precipitated  sulphide  added  to  the  main  quantity 
obtained  from  the  sodium  antimoniate.  The  antimony  sul- 
phide is  best  filtered  through  a  small  tube  in  the  bottom  of 
which  a  little  asbestos  has  been  placed.  The  tube  containing 
the  asbestos  is  gently  heated  over  the  direct  flame  and 
weighed.  When  the  whole  of  the  precipitate  has  been  trans- 
ferred to  the  little  tube,  it  is  warmed  to  expel  the  greater 
portion  of  the  water,  and  then  gently  heated  in  a  stream  of 
dried  carbon  dioxide  until  the  antimony  sulphide  becomes 
black.  The  tube  is  allowed  to  cool  in  a  current  of  the  gas, 
the  carbon  dioxide  displaced  by  atmospheric  air,  and  the  tube 
and  antimony  sulphide  again  weighed. 

The  solution  of  arsenic  sulphide  and  ammonium  carbonate 
is  acidified  with  hydrochloric  acid,  and  the  turbid  solution 
treated  with  a  little  sulphuretted  hydrogen  water,  filtered, 
and  the  arsenic  sulphide  filtered  through  a  weighed  tube 
containing  asbestos,  and  heated  in  the  manner  prescribed  in 
the  case  of  the  antimony  sulphide. 

The  filtrate  and  washings  from  the  original  precipitate  by 
sulphuretted  hydrogen  are  concentrated,  poured  into  a  flask, 
rendered  alkaline  by  ammonia,  and  mixed  with  ammonium 
sulphide.  The  flask,  which  must  be  at  least  half  full  of 
liquid,  is  well  corked  and  allowed  to  stand  24  hours.  When 
the  slight  precipitate  has  subsided  the  liquid  is  filtered,  the 
filtrate  acidified  with  acetic  acid,  and  evaporated  at  a  gentle 
heat  to  facilitate  the  separation  of  a  small  quantity  of  nickel 
sulphide.  This,  mixed  with  sulphur,  is  filtered  off,  slightly 
washed,  and  dried  and  incinerated. 


264  Quantitative  Chemical  A  nalysis. 

The  precipitate  formed  by  the  ammonium  sulphide  is 
treated  on  the  filter  with  a  mixture  of  i  part  hydrochloric 
acid  (sp.  gr.  1-12)  and  6  parts  sulphuretted  hydrogen 
water,  the  filtrate  being  repeatedly  poured  back  on  to  the 
filter.  The  sulphides  of  iron  and  zinc  are  dissolved :  the 
nickel  and  cobalt  sulphides  remain  on  the  filter.  The  filter 
is  dried,  incinerated,  and  mixed  with  the  ash  of  the  filter 
containing  the  nickel  sulphide  derived  from  the  sulphide  of 
ammonium  solution.  The  mixed  ashes  are  treated  with 
aqua  regia,  the  solution  concentrated,  a  little  water  added, 
filtered,  rendered  alkaline  by  ammonia,  a  few  drops  of  am- 
monium carbonate  added,  filtered  into  a  platinum  basin  and 
treated  with  a  few  drops  of  potash  solution  until  no  more 
ammonia  is  evolved.  Filter  off  the  slight  flocculent  pre- 
cipitate, wash,  dry,  ignite,  and  weigh,  and  test  the  precipitate 
(which  generally  consists  mainly  of  nickel  oxide)  for  traces 
of  cobalt  with  the  blowpipe.  The  solution  containing  the 
iron  and  zinc,  to  which  a  few  drops  of  nitric  acid  are 
added,  is  concentrated  by  evaporation,  and  rendered  alka- 
line by  ammonia,  the  ferric  oxide  filtered  off,  again  dissolved 
in  a  few  drops  of  hydrochloric  acid,  and  again  precipitated 
by  ammonia,  filtered,  washed,  dried,  and  weighed.  By  way 
of  control  the  weighed  precipitate  may  be  fused  with  potas- 
sium-hydrogen sulphate,  dissolved  in  water,  and  reduced 
with  zinc,  and  the  iron  estimated  volumetrically  by  a  dilute 
permanganate  solution. 

The  filtrate  from  the  ferric  hydrate  is  mixed  with  a  little 
ammonium  sulphide,  and  allowed  to  stand  at  least  for  24 
hours  at  a  gentle  heat.  If  anything  separates  out  it  is 
filtered  off,  washed,  and  digested  on  the  filter  with  dilute 
acetic  acid  in  order  to  separate  any  admixed  manganese 
sulphide.  The  residue  on  the  filter  is  dried,  and  weighed  as 
zinc  sulphide.  The  acetic  acid  solution  is  concentrated  to  a 
few  cubic  centimetres,  and  mixed  with  excess  of  caustic 
potash  to  precipitate  the  manganese. 

Before  we  can  proceed  to  express  the  results  centesimally 


Refined  Lead.  265 

it  is  necessary  to  determine  the  quantity  of  lead  correspond- 
ing to  the  1,750  cubic  centimetres  of  solution  taken  for 
analysis.  This  can  only  be  estimated  when  we  know  the 
volume  occupied  by  the  lead  sulphate  obtained  from  the  200 
grams  of  metal,  when  suspended  in  water.  By  repeated 
experiments  it  has  been  found  that  it  occupies  the  space 
of  44*99  grams,  or  in  round  numbers,  45  grams  of  wafer 
at  1 6°  C.  The  2-litre  flask,  when  filled  to  the  mark, 
holds  therefore  1,955  cubic  centimetres  solution,  and  45 
cubic  centimetres  lead  sulphate.  But  the  1,955  cubic  cen- 
timetres of  solution  were  equivalent  to  200  grams  lead ; 
therefore  the  1,750  cubic  centimetres  would  be  equal  to 
179-03  grams,  or  in  round  numbers,  179  grams  of  the  original 
lead. 

The  solution  of  the  lead,  when  containing  unusually  large 
quantities  of  antimony,  not  unfrequently  forms,  on  standing, 
a  white  precipitate  of  antimony  oxide  and  antimoniate  of  anti- 
mony, which  occasionally  retains  arsenic.  This  precipitate  is 
filtered  off,  washed,  and  the  arsenic  and  antimony  separated 
as  above.  In  calculating  the  result,  it  must  not  be  forgotten 
that  this  precipitate  is  obtained  from  200  grams  of  the 
metal,  whilst  the  remaining  portion  in  solution  is  assumed 
to  be  derived  from  179  grams  of  lead. 

The  determination  of  the  minute  quantity  of  sulphur  con- 
tained in  lead  may  be  effected  by  heating  the  metal  in 
chlorine  gas,  when  the  sulphur  is  converted  into  the  chloride, 
which,  when  led  into  water,  is  decomposed,  with  the  forma- 
tion of  sulphuric  acid  :  this  may  be  precipitated  in  the 
usual  way,  and  weighed  as  barium  sulphate.  The  best 
method  of  carrying  out  this  process  is  to  heat  about  100 
grams  of  the  lead  in  the  form  of  a  thick  rod  about  i 
centimetre  in  diameter  in  a  combustion-tube  about  i  metre 
long.  In  the  middle  the  tube  is  narrowed,  and  the  end  is 
drawn  out  and  bent  downwards,  and  dips  into  a  small  three- 
bulbed  U-tube  containing  water.  The  lead  is  placed  in  the 
anterior  portion  of  the  tube  ;  the  other  serves  to  collect  the 


266  Quantitative  Chemical  Analysis. 

lead  chloride,  which  flows  over  the  little  bridge  as  fast  as  it 
is  produced,  leaving  the  metal  exposed  to  the  further  action 
of  the  gas.  The  combustion-tube  is  connected  with  a  small 
tube  containing  ignited  fragments  of  charcoal,  over  which 
the  chlorine  passes  before  it  comes  in  contact  with  the 
heated  lead.  The  charcoal  must  be  kept  at  a  red  heat 
throughout  the  operation :  it  serves  to  free  the  chlorine 
from  any  trace  of  accompanying  oxygen,  which  might  oxidise 
the  lead  sulphide  to  sulphate.  Vulcanised  stoppers,  on 
account  of  the  sulphur  they  contain,  cannot  be  used  to 
connect  together  the  several  pieces  of  the  apparatus.  Or- 
dinary corks  must  therefore  be  employed.  When  the 
entire  apparatus  is  filled  with  chlorine  the  lead  is  gradually 
melted,  care  being  taken  to  place  the  combustion-tube  in 
such  a  position  that  the  metal  does  not  come  in  contact  with 
the  cork  •  it  should  flow  against  the  bridge,  but  not  above  it. 
When  the  heat  is  properly  regulated  the  lead  burns  slowly 
to  lead  chloride,  which  flows  over  and  collects  in  the  empty 
portion  of  the  tube.  When  care  is  taken  to  regulate  the 
stream  of  chlorine  and  not  to  overheat  the  chloride,  but 
little  of  this  substance  passes  over  into  the  U-tube.  The 
solution  is  washed  out  of  the  condensing  tube  into  a  small 
beaker,  heated,  and  the  sulphur  precipitated  by  the  addition 
of  a  few  drops  of  barium  chloride.*  The  following  analysis 
of  three  specimens  of  soft  lead,  executed  by  the  above 
methods,  will  give  some  idea  of  the  nature  and  amount  of 
its  impurities :  the  results  are  represented  centesimally  : — 

0-00385 
•00190 

•00553 
•02639 
•00129 


*  Bannow  and  Kramer,  «  Ber.  Deutschen  Chem.  Gesells.'    July  1871. 


Silver  . 

0-0020® 

O  '0006  2 

Copper 

•00228 

•OOO3I 

Cadmium 

trace 

•oooio 

Bismuth 

•00040 

•oooio 

Antimony 

•00173 

•00186 

Iron    . 

•00035 

•00012 

Zinc    . 

•00014 

•00023 

Sulphur 

•00076 

•00008 

White  Lead.  267 


XXX.     WHITE  LEAD. 

This  substance  is  a  compound  of  lead  carbonate  and 
hydrate  in  variable  proportions.  In  general  the  relation 
between  the  hydrate  and  carbonate  may  be  represented  by 
the  formula  2PbCO3  +  PbH2O2,  although  specimens  of  the 
composition  3PbCO3  +  PbH2O2  and  5PbCO3  +  PbH2O2 
are  occasionally  obtained  (Mulder,  Phillips).  As  found  in 
commerce  it  is  frequently  mixed  with  barium  sulphate  (heavy 
spar),  barium  carbonate  (witherite),  calcium  carbonate,  and 
zinc  oxide.  These  bodies  cannot  always  be  regarded  as 
adulterants ;  the  heavy  spar,  for  example,  serves  to  protect 
the  lead  from  the  rapid  action  of  sulphuretted  hydrogen, 
and  unless  present  in  large  excess  does  not  interfere  with 
the  opacity  or  body  of  the  pigment.  But  these  substances 
are  not  unfrequently  added  in  undue  quantity  ;  and  perfectly 
pure  white  lead  is  now  comparatively  rare  as  an  article  of 
commerce. 

Determination  of  the  Carbon  Dioxide. — From  i  to  2  grams 
of  the  substance  are  weighed  out  into  the  flask  A,  fig.  31,  and 
decomposed  by  moderately- dilute  nitric  acid.  The  operation 
is  carried  out  exactly  in  the  manner  described  on  p.  86. 

On  the  termination  of  the  experiment  the  liquid  in  the 
flask  A  is  filtered  if  necessary,  and  the  residue  washed,  dried, 
and  weighed :  it  may  consist  of  the  sulphates  of  barium  or  cal- 
cium. The  weighed  residue  is  boiled  in  a  platinum  or  porcelain 
basin  with  solution  of  pure  sodium  carbonate  for  an  hour  or 
so,  care  of  course  being  taken  to  replenish  the  dish  with  water 
from  time  to  time.  Any  calcium  sulphate  present  is  com- 
pletely decomposed,  and,  on  pouring  the  liquid  through  a 
small  filter,  the  sulphate  in  solution  may  be  detected  by  the 
addition  of  barium  chloride.  The  calcium  carbonate  formed 
may  be  dissolved  out  by  dilute  hydrochloric  acid,  and  the  lime 
precipitated  by  means  of  ammonium  oxalate.  The  washed 
precipitate  is  rendered  caustic  by  ignition  and  weighed. 


268  Quantitative  Chemical  Analysis. 

The  residual  barium  sulphate  may  also  be  weighed,  by  way 
of  control. 

The  nitric  acid  nitrate  containing  the  lead,  &c.,  is  evapo- 
rated nearly  to  dryness  to  expel  the  excess  of  the  acid, 
diluted  with  water,  and  the  liquid  saturated  with  sulphuretted 
hydrogen.  The  lead  is  completely  separated  ;  it  is  filtered 
off,  and  converted  into  sulphate  by  oxidation  with  nitric 
acid.  The  weighed  precipitate  should  then  be  heated  with 
a  dilute  solution  of  sodium  thiosulphate,  which  dissolves  the 
lead  sulphate  and  leaves  unattacked  any  barium  sulphate 
which  may  be  present.  To  the  filtrate  from  the  lead  sulphide 
add  ammonia  and  ammonium  sulphide ;  wash  the  precipi- 
tated zinc  sulphide,  dissolve  it  in  dilute  hydrochloric  acid,  and 
re-precipitate  as  carbonate,  and  convert  into  oxide  by  ignition. 
The  filtrate  from  the  zinc  sulphide  contains  the  lime  and 
baryta :  these  are  separated  as  in  No.  VII.  Part  II.  If  baryta 
is  found  no  calcium  sulphate  can  be  present.  If  it  is  desired 
to  determine  the  water  directly,  this  can  be  effected  by  means 
of  the  apparatus  shown  in  fig.  53. 

Arrangement  of  the  Results. — Calculate  the  baryta  found 
in  the  last  filtrate  to  barium  carbonate,  and  the  lime  to 
sulphate  or  carbonate  according  to  circumstances.  The 
residual  amount  of  carbon  dioxide  is  converted  into  lead 
carbonate,  and  the  remainder  of  the  lead  to  lead  oxide. 
The  water,  zinc  oxide,  and  barium  sulphate  are  set  down  as 
such  in  the  statement  of  the  analysis. 

Zinc  white  may  be  also  analysed  by  the  foregoing  method. 
In  addition  to  the  adulterants  mentioned  above,  kaolin  is  not 
unfrequently  met  with  :  this  is  left  undissolved  on  treating 
the  pigment  with  dilute  acid. 

XXXI.     CHROME  IRON-ORE. 

This  mineral  occurs  massive  in  various  parts  of  the  world, 
particularly  in  Norway,  Siberia,  Asia  Minor,  Silesia,  and 
North  America ;  it  consists  essentially  of  a  compound  of 


Chrome  Iron-Ore.  269 

ferrous  oxide  and  chromium  sesquioxide.  It  belongs  to  the 
spinelle  group  of  minerals,  and  is  isomorphous  with  magnetic 
oxide  of  iron.  Its  formula,  FeOCr2O3,  requires  677  per 
cent,  of  chromium  sesquioxide.  Usually,  however,  the  chro- 
mium is  replaced  by  aluminium,  and  the  iron  by  magnesium 
to  a  considerable  extent,  and  the  ore  is  of  very  good  quality 
when  it  contains  50  per  cent,  of  chromium  sesquioxide. 

Determination  of  the  Chromium  Sesquioxide. — Grind  a  few 
grams  of  the  carefully- sampled  mineral  in  an  agate  mortar, 
and  pass  the  powder  through  a  fine  muslin  sieve.  When 
you  have  collected  about  two  grams  of  the  fine  dust,  again 
grind  it  in  portions  of  a  decigram  at  a  time  in  the  agate  mortar 
until  every  feeling  of  grittiness  has  disappeared,  and  the  ore 
cakes  in  an  impalpable  powder  round  the  pestle.  Too  much 
care  cannot  be  given  to  the  grinding  :  the  success  of  the 
analysis  entirely  depends  upon  the  ore  being  in  the  finest 
possible  state  of  division.  Weigh  out  about  0-5  gram  of 
the  powder  into  a  platinum  crucible  of  about  50  cubic  centi- 
metres capacity,  and  place  over  it  12  times  its  weight  of 
recently-fused  potassium-hydrogen  sulphate,  and  gently  heat 
the  crucible  so  as  merely  to  melt  the  sulphate.  Keep  it 
melted  at  a  gentle  heat  for  15  or  20  minutes,  and  gradually 
increase  the  temperature  until  the  bottom  of  the  crucible 
becomes  red  hot.  Care  must  be  taken  that  the  fused  mass 
does,  not  rise  above  half  way  up  the  crucible.  In  a  few 
minutes  the  mixture  will  fuse  quietly  and  dense  fumes  of 
sulphuric  acid  will  be  evolved;  the  heat  is  now  gradually 
increased  until  the  crucible  is  at  a  bright  red  heat ;  in  about 
half  an  hour  remove  the  lamp,  and  add  about  6  parts  of 
powdered  sodium  carbonate,  again  fuse  the  mixture,  and, 
keeping  the  temperature  for  an  hour  at  a  red  heat,  add  little 
by  little  the  same  quantity  of  nitre.  The  temperatui 
crucible  is  now  increased  and  kept  at  a  full  red  JJH&;  for  20 
minutes;  it  is  allowed  to  cool,  transferred  to  a  porcelain  basin, 
and  the  mass  boiled  out  with  water.  The  solution  is  filtered, 


270  Quantitative  Chemical  Analysis. 

and  the  residue  washed  with  hot  water  until  the  filtrate  comes 
through  colourless.  It  is  not  necessary  to  transfer  the  whole 
of  the  insoluble  matter  to  the  filter.  Quickly  dry  the  filter 
and  its  contents,  detach  the  ferric  oxide  and  return  it  to  the 
porcelain  basin,  burn  the  filter  and  add  the  ash,  and  digest 
the  whole  at  a  gentle  heat  with  moderately-concentrated 
hydrochloric  acid.  If  the  fusion  has  been  properly  con- 
ducted the  residue  will  dissolve  ;  any  black  insoluble  matter 
left  undissolved  denotes  that  the  grinding  has  not  been  done 
with  sufficient  care.  This  insoluble  portion  must  be  collected 
on  a  small  filter,  dried,  and  the  filter,  &c.,  folded  up,  thrown 
into  the  platinum  crucible,  ignited,  and  the  residue  again 
fused  with  potassium  bisulphate,  sodium  carbonate,  and  nitre, 
and  the  fused  mass  again  boiled  out  with  water,  filtered,  and 
the  filtrate  added  to  the  main  solution.  A  few  grams  of 
ammonium  nitrate  are  added  to  the  total  filtrate,  and  the 
liquid  is  evaporated  nearly  to  dryness,  water  is  added,  and 
the  solution  is  filtered  from  the  alumina,  silica,  &c.  The  fil- 
trate, which  should  be  received  in  a  porcelain  basin,*  is  then 
made  strongly  acid  with  sulphurous  acid,  and  boiled  until 
.  the  gas  is  nearly  expelled,  a  slight  excess  of  ammonia  is 
added,  and  the  solution  is  again  boiled  until  it  becomes 
colourless.  Pour  the  liquid  on  to  a  filter,  wash  the  precipi- 
tate by  decantation  with  hot  water,  and  by  means  of  a  feather 
transfer  it  to  the  filter,  and  wash  carefully  with  hot  water. 
After  the  sixth  washing  allow  the  filter  and  precipitate  to 
drain  thoroughly  by  keeping  up  the  action  of  the  pump  for 
about  ten  minutes,  remove  the  filter,  fold  it,  and,  without 
further  drying,  transfer  it  to  a  weighed  platinum  crucible 
and  cautiously  heat  with  the  lid  on.  Gradually  increase  the 
heat,  and  as  soon  as  the  paper  is  charred,  remove  the  lid, 
placing  it  at  the  edge  of  the  crucible  (see  fig.  15),  and  ignite 


tiling  with  ammonia  is  conducted  in  glass  vessels  there  is 
great  probabfll^  that  the  precipitated  chromic  oxide  will  be  contaminated 
with  silica.  Trie  error  from  this  cause  may  amount  to  0-5  per  cent. 
(Compare  Souchay,  Fres.,  'Zeits.'  iv.  66.) 


Chrome  Iron  Ore.  271 

strongly  for  10  or  15  minutes.  If  the  precipitate  has  been 
drained  sufficiently  by  the  action  of  the  pump,  there  is  no 
danger  of  the  oxide  being  projected  from  the  crucible  on 
ignition.  On  treating  the  weighed  precipitate  with  a  few 
drops  of  water  the  liquid  ought  to  remain  colourless :  a  yellow 
colour  indicates  that  the  oxide  has  been  imperfectly  washed 
from  alkaline  salts. 

Complete  analysis  of  chrome  iron  ore  (Dittmar's  process). — 
Fuse  together  a  mixture  of  equal  weights  of  pure  borax  glass 
and  sodium  potassium  carbonate  in  a  platinum  basin,  and 
break  up  the  fused  mass  when  cold.  10  grams  of  this 
mixture  are  placed  in  a  platinum  crucible  of  about  50  c.c. 
capacity,  fused,  and  allowed  to  cool.  About  i  gram  of  the 
finely  powdered  ore  is  added,  and  the  mass  again  fused,  at 
first  with  the  lid  on  :  the  crucible  is  now  inclined,  and  the 
contents  are  stirred  with  a  stout  platinum  wire.  In  about 
15  minutes  the  whole  of  the  ore  should  be  dissolved.  The 
lid  is  now  placed  in  the  position  seen  in  fig.  15,  the  fused 
mass  being  occasionally  stirred  to  promote  its  oxidation  : 
this  is  usually  complete  in  about  half  an  hour.  Allow  to 
cool,  and  add  about  5  grams  of  pure  potassium  carbonate, 
again  fuse,  and  pour  out  the  melted  mass  into  a  platinum 
basin,  which  should  be  quickly  covered  with  a  clock  glass 
as  the  solidifying  substance  decrepitates.  The  portion  ad- 
hering to  the  crucible  is  dissolved  off  by  hot  water  in  a 
porcelain  basin,  and  the  solidified  mass  in  the  dish  is  added 
to  the  solution.  Add  a  few  drops  of  alcohol  to  reduce  any 
manganate  which  may  be  present,  and  digest  on  a  water- 
bath  until  the  whole  is  disintegrated  and  the  alcohol  ex- 
pelled. Filter  and  wash  the  precipitate  until  the  filtrate  is 
colourless,  and  dissolve  the  precipitate  in  hot  dilute  sul- 
phuric acid  to  ascertain  if  any  ore  is  left  undecomposed. 
If  any  be  found  it  must  be  treated  again  with  a  small  quan- 
tity of  the  fusion  mixture,  and  the  mass  dissolved  out  with 
water  as  before.  The  filtrate  is  concentrated  to  about 
200  c.c.,  and  divided  by  weighing  or  measuring  into  two 


272  Quantitative  Chemical  A  nalysis. 

approximately  equal  portions.  To  the  one  portion  is  added 
a  quantity  of  dilute  sulphuric  acid  and  a  known  weight  (in 
excess)  of  pure  ferrous  sulphate,  and  the  amount  of  un- 
oxidised  iron  determined  by  standard  potassium  bichromate 
solution  as  directed  on  p.  221. 

Cr2O3  :  6Fe  =  152-2  :  336  =  -4529  :  i. 

If  only  the  amount  of  chromium  is  required,  the  determina- 
tion may  be  repeated  on  the  second  portion.  To  determine 
the  amount  of  silica  and  alumina,  add  ammonium  nitrate  to 
the  solution,  evaporate  to  dryness,  add  water,  and  filter  off 
the  alumina  and  silica. 

The  sulphuric  acid  solution  is  treated  with  sulphuretted 
hydrogen,  filtered,  and  the  filtrate  concentrated  to  a  small 
bulk,  and  poured  into  an  excess  of  strong  and  fresh  potash 
solution  contained  in  a  platinum  basin,  filtered,  and  the 
alumina  and  small  quantity  of  silica  precipitated  by  boiling 
with  ammonium  chloride :  the  precipitate  is  added  to  that 
obtained  as  above,  and  the  substances  are  separated  as  in 
an  ordinary  silicate  analysis.  The  separation  of  iron,,  lime, 
and  magnesia  is  effected  as  described  on  p.  87. 

XXXII.     SMALTINE:  COBALT-GLANCE. 

Smaltine  or  tin-white  cobalt  is  an  arsenide  of  cobalt : 
cobalt-glance  is  essentially  a  sulpharseniate  of  cobalt.  Both 
minerals  frequently  contain,  in  addition  to  cobalt,  arsenic, 
and  sulphur,  variable  quantities  of  nickel,  iron,  lead,  bismuth, 
copper  and  gangue. 

Determination  of 'Sulphur. — See  Copper  Pyrites,  p.  211. 

Determination  of  Silica  and  the  Metals. — Treat  2  grams  of 
the  finely-divided  substance  with  strong  nitric  acid,  and 
evaporate  to  dryness  with  sulphuric  acid  (hydrochloric  acid 
is  inadmissible,  since  a  small  quantity  of  the  arsenic  would 
be  volatilised  on  heating).  Moisten  the  dried  mass  with 
sulphuric  acid  and  add  water.  On  standing,  the  supernatant 
liquid  ought  to  become  quite  clear :  a  turbidity  indicates  the 


Smaltine :  Cobalt-Glance.  273 

presence  of  basic  salts.  Filter  the  liquid  into  a  flask,  wash 
the  precipitate  slightly  with  water  containing  sulphuric  acid, 
remove  the  acid  from  the  paper  by  alcohol,  dry,  and  weigh ; 
the  insoluble  matter  consists  of  silica,  and  calcium  and  lead 
sulphates.  Transfer  the  weighed  precipitate  to  a  small  beaker 
and  boil  with  dilute  nitric  acid  :  the  sulphates  are  thus  dis- 
solved. Throw  the  residual  silica  on  a  filter,  wash,  dry,  and 
weigh.  The  nitrate  containing  the  lead  and  calcium  is  evapo- 
rated to  dry  ness  with  hydrochloric  acid,  water  added,  the 
solution  boiled,  and  the  lead  precipitated  with  sulphuretted 
hydrogen  and  converted  into  sulphate  by  treatment  with 
strong  nitric  acid.  The  lead  sulphate  is  weighed :  its  weight, 
plus  that  of  the  silica,  subtracted  from  the  original  weight  of 
the  precipitate  (SiO2  +  PbSO4  +  CaSO4),  gives  the  cal- 
cium sulphate. 

To  the  filtrate  from  the  insoluble  residue  (SiO2.PbSO4. 
CaSO4)  a  strong  solution  of  sulphurous  acid  is  added  in 
small  quantities  at  a  time,  and  the  liquid  boiled  after  each 
addition.  The  liquid  is  maintained  at  a  temperature  of 
60°  or  70°  by  surrounding  it  with  warm  water,  and  the 
metals  precipitated  by  sulphuretted  hydrogen.  Allow  the 
solution  to  stand  for  some  time,  so  that  the  greater  portion 
of  the  sulphuretted  hydrogen  may  be  removed  by  diffusion, 
and  filter  the  liquid.  Wash  and  drain  the  filter  thoroughly 
by  the  action  of  the  pump,  spread  it  on  a  glass  plate,  and 
remove  the  arsenic  sulphide  by  means  of  a  thick  platinum 
wire,  transferring  it  to  a  large  porcelain  crucible.  Fold  the 
filter  and  replace  it  in  the  funnel,  and  dissolve  the  small 
quantity  of  adhering  sulphide  by  a  few  drops  of  strong 
potash  solution,  and  allow  the  liquid  to  fall  into  the  porcelain 
crucible.  A  little  more  potash  is  added  to  the  crucible  until 
the  arsenic  sulphide  is  dissolved.  A  stream  of  chlorine  is 
then  passed  into  the  solution  until  it  is  perfectly  colourless : 
it  is  then  gently  heated  for  some  time,  and  filtered  from  the 
small  quantities  of  the  oxides  of  copper  and  bismuth.  Dis- 
solve the  residue  in  nitric  acid,  add  an  excess  of  ammonium 
carbonate,  heat  gently  for  some  time\  filter,  redissolve  in 

T 


274  Quantitative  Chemical  Analysis. 

nitric  acid,  and  again  precipitate  the  bismuth  by  the  addition 
of  ammonium  carbonate  in  excess,  dry,  and  weigh  the 
bismuth  as  trioxide.  Mix  the  two  filtrates  and  precipitate 
the  copper  by  means  of  caustic  potash. 

The  filtrate  containing  the  arseniate  of  potassium  is  acidi- 
fied with  hydrochloric  acid,  and  ammonia,  and  *  magnesium 
mixture'  added.  The  magnesium-ammonium  arseniate  may 
be  obtained  highly  crystalline  (in  which  state  it  washes 
better),  by  first  adding  the  magnesia  mixture  to  the  acid 
liquid,  and  then  a  large  quantity  of  ammonia — about  one- 
fourth  of  the  bulk  of  the  entire  liquid.  After  standing  24 
hours,  pour  the  liquid  on  to  a  weighed  filter,  wash  the  pre- 
cipitate with  ammonia  water  until  the  washings  acidified 
with  nkric  acid  give  only  a  slight  opalescence  with  silver 
nitrate.  The  magnesium-ammonium  arseniate  is  dried  in 
the  air-bath  at  120°,  and  ignited  at  a  low  red  heat  in  a 
weighed  porcelain  crucible.  The  ignited  residue  has  the 
composition  Mg2As2O7,  corresponding  ta  magnesium  pyro- 
phosphate. 

To  the  rose-coloured  filtrate  containing  the  cobalt,  nickel, 
and  iron,  add  nitric  acid,  and  boil,  and  then  solid  sodium 
carbonate,  until  the  liquid  is  nearly  neutralised,  and  becomes 
slightly  turbid,  owing  to  the  separation  of  ferric  hydrate : 
complete  the  precipitation  by  adding  a  solution  of  succinate 
of  soda  or  ammonia,  wash  and  dry  the  precipitate,  ignite  it, 
and  weigh  it  as  Fe2O3. 

The  nickel  and  cobalt  are  co-precipitated  by  adding 
potash  to  the  boiling  solution ;  the  precipitate  is  filtered  off 
and  washed,,  drained  as  far  as  practicable  by  the  pump,  the 
filter  spread  on  a  plate,  and  the  precipitate,  detached  as  far 
as  possible  by  means  of  a  thick  platinum  wire,  transferred  to 
a  small  porcelain  dish :  the  filter  is  re-folded  and  dropped 
back  again  into  the  funnel.  Treat  the  precipitate  in  the  dish 
with  dilute  hydrocyanic  acid,  and  then  with  potash  solution, 
and  again  with  hydrocyanic  acid,  and  warm  on  the  water- 
bath,  until  no  further  solution  occurs.  A  minute  quantity 
of  substance  frequently  remains  undissolved :  it  consists  of 


Smaltine :  Cobalt- Glance. 


275 


paracyanogen,  and  retains  a  small  quantity  of  the  mixed 
oxides.  This  is  added  to  the  small  quantity  remaining  on 
the  filter,  which  is  now  washed,  dried,  ignited,  and  weighed. 
The  proportion  of  the  two  metals  contained  in  it  is  calculated 
from  the  results  of  the  after-separation.  The  solution  of 
nickel  and  cobalt  is  boiled  to  expel  the  excess  of  acid  :  it  is 
reddish-yellow,  and  consists  now  of  cobalticyanide  of  potas- 
sium and  double  cyanide  of  nickel  and  potassium.  Add  to 
the  hot  solution  precipitated  (yellow)  mercuric  oxide,  and 

FIG    64. 


again  boil  for  some  time.  All  the  nickel  is  precipitated, 
partly  as  cyanide,  partly  as  sesquioxide,  the  mercury  com- 
bining with  the  liberated  cyanogen.  Wash  the  precipitate 
thoroughly,  dry  it,  and  heat  it  for  some  time  in  a  covered 
porcelain  crucible  to  expel  the  excess  of  mercuric  oxide. 
The  filtrate  is  nearly  neutralised  with  nitric  acid,  and  a  neutral 
solution  of  mercurous  nitrate  is  added  in  excess,  when 
cobalticyanide  of  mercury  is  precipitated ;  this  is  washed, 
dried,  and  ignited,  in  an  open  porcelain  crucible,  until  the 
weight  is  constant :  it  has  the  composition  Co3O4. 


276  Quantitative  Chemical  Analysis. 

By  way  of  control  it  may  be  reduced  to  the  metallic  state, 
by  heating  in  a  stream  of  dry  hydrogen.  When  the  decom- 
position is  apparently  finished,  allow  the  metal  to  cool  in  a 
current  of  the  gas  and  weigh  :  again  heat  in  hydrogen  and 
again  weigh,  repeating  the  operation  until  the  weight  is  con- 
stant. The  apparatus  employed  for  the  reduction  is  seen 
in  fig.  64.  The  crucible  lid  is  pierced  with  a  hole  and  pro- 
vided with  a  porcelain  tube. 


XXXIII.     FAHL-ORE  (TETRAHEDRITE) 

Consists  of  a  mixture  of  sulphantimonites  and  sulpharsenites 
of  copper,  silver,  iron,  zinc,  and  mercury.  Every  specimen 
of  the  oreY however,  does  not  contain  all  these  substances :  in 
some  the^silver,  zinc,  and  mercury  are  entirely  absent ;  mothers 
the  whole  ofMhe  arsenic  is  replaced  by  antimony.  The  com- 
position of  the  mineral  maybe  represented  by  the  general 

formula    ^3  :  N"'  4\  S7,  in  which  M  denotes  Cu  and  Ag, 

oD2     I     ASa-* 

and  N"  denotes  Fe,  Zn,  and  Hg. 

Determination  of  the  Sulphur. — See  Copper  Pyrites,  p.  209. 

Determination  of  the  Metals. — The  decomposition  of  the 
finely-powdered  mineral  is  best  accomplished  by  heating  it 
in  a  stream  of  dry  chlorine.  The  apparatus  required  for 
this  purpose  is  seen  in  fig.  64A.  The  substance  is  placed  in 
the  bulb-tube  a-,  the  second  bulb  serves  to  collect  the 
greater  portion  of  the  sublimate,  and  thus  prevents  the 
narrower  portion  from  being  stopped  up  by  the  chlorides. 
The  mineral  is  best  weighed  out  from  a  tube  of  a  diameter 
sufficiently  narrow  to  allow  of  its  being  introduced  into  the 
bulb-tube.  The  bulb-tube  is  connected  with  the  U-tube  b, 
which  is  filled  to  the  extent  of  ^  of  the  two  upper  bulbs  with 
a  mixture  of  hydrochloric  and  tartaric  acids.  The  flask  A 
contains  the  chlorine  mixture  (i  part  of  salt,  i  of  manganese 
dioxide,  i  of  water,  and  2  of  sulphuric  acid :  the  sulphuric  acid 


Fdhl-Ore. 


277 


and  water  are  previously  mixed,  and  allowed  to  cool) ;  the 
small  flask  c  contains  strong  sulphuric  acid,  and  the  U-tube  d 
sulphuric  acid  and  pumice.  Allow  this  portion  of  the  ap- 
paratus to  become  rilled  with  chlorine  before  joining  it  to 
the  bulb-tube,  and  do  not  heat  the  mineral  until  the  gas  is 
apparently  without  further  action  upon  it.  Indeed  as  soon 
as  the  chlorine  comes  in  contact  with  the  fahl-ore  immediate 
signs  of  decomposition  ensue,  and  the  bulb  becomes  very 
hot.  When  it  cools,  gently  heat  it  in  order  to  drive  off  the 

FIG.  64  A. 


volatile  products  into  the  second  bulb.  Care  must  be  taken 
to  send  only  a  gentle  stream  of  chlorine  through  the  appara- 
tus, otherwise  there  is  danger  of  a  portion  of  the  sublimate 
passing  through  the  liquid  unabsorbed.  As  soon  as  reddish 
vapours  of  ferric  chloride  make  their  appearance,  discontinue 
the  heating,  and  allow  the  apparatus  to  cool.  Divide  the 
bulb-tube  a  by  means  of  a  file  between  the  two  bulbs,  and 
allow  the  portion  containing  the  sublimate  to  remain  in  a 
damp  place  for  24  hours,  in  order  that  it  may  take  up 


278  Quantitative  Chemical  Analysis. 

moisture  from  the  atmosphere  :  this  prevents  the  evolution 
of  the  great  heat  and  consequent  possibility  of  loss  by  vola- 
tilisation, when  the  chlorides  are  subsequently  treated  with 
water.  In  the  meantime  proceed  with  the  analysis  of  the 
fixed  residue  contained  in  the  first  bulb ;  this  contains  the 
silver,  copper,  zinc,  and  a  portion  of  the  iron.  It  is  placed 
in  a  beaker  and  digested  with  dilute  hydrochloric  acid  until 
everything  is  dissolved,  with  the  exception  of  the  silver  chlo- 
ride. This  is  filtered  off,  washed  with  hot  water,  dried,  and 
weighed.  The  copper  in  the  filtrate  is  precipitated  by  sul- 
phuretted hydrogen,  re-dissolved  in  nitric  acid,  and  weighed 
as  oxide.  The  solution  containing  the  zinc  and  iron  is 
boiled  with  nitric  acid  for  a  short  time,  and  the  metals 
separated  as  in  No.  XIII.  Part  II. 

The  solution  of  hydrochloric  and  tartaric  acids  is  poured 
into  a  beaker,  the  tube  rinsed  out,  and  the  bulb  containing 
the  sublimate  added.  This  solution  contains  the  arsenic,  anti- 
mony, mercury,  and  the  remainder  of  the  iron ;  if  it  is  cloudy, 
from  the  separation  of  a  little  antimony,  warm  \\.  gently,  some- 
times the  turbidity  is  due  to  the  separation  of  sulphur ;  in  that 
case  the  liquid  should  be  filtered.  The  solution  is  heated 
to  about  60°,  and  a  current  of  sulphuretted  hydrogen  is 
passed  through  it  for  some  time  until  the  fluid  smells  strongly 
of  the  gas.  It  is  allowed  to  stand  for  12  or  15  hours,  filtered, 
and  washed  with  sulphuretted  hydrogen  water.  The  re- 
mainder of  the  iron  is  found  in  the  filtrate  ;  it  is  precipitated 
by  ammonium  sulphide,  washed  with  sulphuretted  hydrogen 
water,  re-dissolved  in  hydrochloric  acid,  the  solution  boiled 
with  a  little  nitric  acid  or  a  few  crystals  of  potassium  chlo- 
rate, and  the  iron  re-precipitated  as  ferric  oxide  by  ammonia, 
washed,  dried,  and  weighed. 

The  mixed  sulphides  of  antimony,  arsenic,  and  mercury 
are  treated  with  potassium  sulphide  until  the  residue  of 
mercuric  sulphide  is  quite  black.  This  is  filtered  off  through 
a  weighed  filter,  washed  with  water  containing  potassium 
sulphide,  then  .with  pure  water,  two  or  three  times  with 
alcohol,  and  finally  with  bisulphide  of  carbon,  until  a  drop 


Silver :  Pisani's  Method.  279 

of  the  filtrate  evaporated  on  a  watch-glass  leaves  no  residue. 
The  filter  is  once  more  dried  at  100°  and  weighed. 

The  sulphide  of  potassium  solution  is  transferred  to  a 
porcelain  crucible  and  saturated  with  chlorine  gas.  The 
solution  is  heated  on  a  water-bath,  and  mixed  with  a  con- 
siderable excess  of  hydrochloric  acid  and  evaporated  to 
about  half  its  bulk  ;  an  equal  volume  of  hydrochloric  acid 
is  again  added  to  the  solution,  again  evaporated  to  one-half. 
A  freshly  prepared  solution  of  sulphuretted  hydrogen  is  next 
added  (in  the  proportion  of  TOO  c.c.  of  solution  to  each  'i 
gram  of  antimonic  acid  supposed  to  be  present),  when  anti- 
mony pentasulphide  separates  out.  The  excess  of  sulphu- 
retted hydrogen  is  quickly  expelled  by  blowing  a  stream  of 
air  through  the  liquid,  and  the  precipitate  is  brought  upon  a 
weighed  filter,  drained  by  means  of  the  pump,  and  washed 
six  or  eight  times  with  alcohol,  then  with  carbon  bisulphide, 
and  again  with  alcohol.  The  antimony  pentasulphide  is 
dried  at  110°,  and  weighed. 

The  aqueous  filtrate  is  mixed  with  a  few  drops  of  chlorine 
water,  heated  on  the  water- bath,  and  treated  with  a  stream 
of  sulphuretted  hydrogen,  and  after  standing  for  24  hours  in 
a  warm  place  the  precipitated  arsenic  pentasulphide  is 
transferred  to  a  weighed  filter,  and  washed  with  alcohol  and 
carbon  bisulphide,  as  in  the  case  of  the  antimony  sulphide, 
and  treated  at  110°  until  its  weight  is  constant. 

Bournonite(2PbSCuS.Sb2S3);  Boulangerite  (3PbS.Sb2S3); 
red  silver  ore,  3Ag2SAs2(Sb2)S3,  and  nickelspeiss  may  also 
be  analysed  by  decomposition  in  a  stream  of  chlorine. 

XXXIV.     DETERMINATION  OF  SILVER  IN  SOLUTIONS. 

An  excess  of  a  standard  solution  of  sodium  chloride  is 
mixed  with  a  determinate  volume  of  the  silver  solution  to  be 
tested,  a  few  drops  of  potassium  chromate  solution  are  added, 
and  the  excess  of  the  chlorine  in  solution  is  determined  by 
adding  a  solution  of  silver  of  known  strength  until  the  orange 
colour  of  the  silver  chromate  is  persistent  (compare  No.  I. 


280  Quantitative  Chemical  Analysis. 

Part  III.).  This  method  is  especially  applicable  to  the  deter- 
mination of  the  strength  of  the  silver  solutions  employed  in 
photography. 

Pisanfs  Method  (particularly  applicable  to  the  Estimation  of 
Silver  in  Alloys  and  Ores). 

If  a  solution  of  the  blue  compound  of  iodine  and  starch 
be  added  to  a  neutral  liquid  containing  silver  nitrate,  the 
colour  is  rapidly  destroyed,  the  iodine  combining  with  the 
silver  to  form  silver  iodide  (and  iodate  ?).  Immediately  the 
silver  is  completely  precipitated  the  iodised  starch  solution 
colours  the  liquid  permanently  blue,  and  thus  marks  the 
completion  of  the  process. 

This  method  is,  of  course,  only  accurate  in  the  absence  of 
substances,  other  than  silver,  which  effect  decomposition  of 
the  blue  solution.  Tin,  arsenic,  and  antimony,  mercury,  iron, 
and  manganese  protoxides  and  gold  must  accordingly  be 
absent.  Copper  and  lead  do  not  influence  the  reaction. 

Weigh  out  about  2  grams  of  iodine  and  15  grams  of  pure 
starch  into  a  porcelain  mortar,  add  6  or  8  drops  of  water, 
and  mix  intimately ;  transfer  the  mass  to  a  flask,  and  heat  it, 
well  closed,  on  the  water-bath  for  an  hour.  The  violet-blue 
colour  of  the  mixture  will  now  have  changed  to  dark  greyish- 
blue.  Dissolve  in  water  and  dilute  considerably. 

To  ascertain  the  value  of  the  deep  bluish-black  solution, 
transfer  10  c.c.  of  a  neutral  solution  of  silver  nitrate,  contain- 
ing i  gram  of  silver  per  litre,  to  a  beaker,  add  a  small  quan- 
tity of  pure  precipitated  calcium  carbonate,  and  run  in  the 
solution  of  iodised  starch,  with  constant  stirring,  until  the 
yellow  colour  of  the  liquid  (due  to  the  silver  iodide  formed) 
changes  to  greenish  -  blue.  The  10  c.c.  of  silver  solution 
should  require  about  50  c.c.  of  the  iodised  starch  solution. 
The  object  of  the  calcium  carbonate  is  to  neutralise  the 
nitric  acid  liberated:  it  also  renders  the  completion  of  the 
reaction  more  distinctly  visible. 

The  minute  quantities  of  silver  contained  in  lead  ores  and 


Silver  Assay.  281 

in  refined  lead  may  be  readily  estimated  by  this  method. 
The  nitric  acid  solution,  prepared  as  directed  on  p.  259, 
is  mixed  with  sulphuric  acid  to  remove  the  lead,  the 
liquid  is  filtered,  mixed  with  calcium  carbonate  in  excess, 
and  again  filtered.  A  small  additional  quantity  of  the  cal- 
cium carbonate  is  added  to  the  filtrate,  which  is  titrated  with 
the  standardised  iodine  solution  in  the  manner  above  de- 
scribed. 

A  determinate  volume  of  the  standard  iodine  solution, 
p.  156,  mixed  with  clear  starch  liquor,  and  diluted  to  centi- 
normal  strength,  may  be  employed  with  equally  good  results. 
(Field.) 

XXXV.    ASSAY  OF  SILVER  IN  BULLION,  COIN,  PLATE,  &c. 

The  method  about  to  be  described  is  that  generally  prac- 
tised in  the  European  mints  :  it  was  originally  devised  by 
Gay-Lussac,  and  has  been  rigidly  investigated  by  Mulder, 
Probably  no  quantitative  process  is  susceptible  of  such  a 
high  degree  of  accuracy  as  the  estimation  of  silver  by  the 
*  humid  method,'  as  the  process  of  Gay-Lussac  is  generally 
termed,  in  contradistinction  to  the  older  process  of  cupella- 
tion. 

We  have  already  indicated  the  leading  features  of  this 
method  in  describing  the  process  for  exactly  estimating  the 
strength  of  a  standard  hydrochloric  acid  solution  (p.  131). 
This  very  simple  process  is,  however,  complicated  by  the 
following  circumstance  :  If  we  add  i  eq.  of  silver  nitrate  to 
i  eq.  of  sodium  chloride,  both  dissolved  in  water,  we  should 
expect  that  all  the  silver  would  be  precipitated,  and  that  we 
should  obtain  no  subsequent  turbidity  by  the  further  addi- 
tion either  of  salt  or  of  silver  solution.  But  in  reality  we 
find  that  the  addition  of  either  of  the  solutions  produces  a 
turbidity.  This  remarkable  fact  probably  depends  upon 
the  solvent  action  of  the  sodium  nitrate,  and  upon  the  ex- 
istence of  a  certain  degree  of  equilibrium  between  the  silver 
nitrate  and  common  salt,  which  is  destroyed,  with  the  im- 


282  Quantitative  Chemical  Analysis. 

mediate  formation  of  silver  chloride,  by  the  addition  of  either 
of  the  bodies. 

It  thus  happens  that  if  we  add  a  decimal  solution  of  salt 
(vide  infra),  drop  by  drop,  to  a  solution  of  silver  until  no 
further  turbidity  is  produced,  and  then  add  decimal  solution 
of  silver  to  the  liquid,  we  again  notice  the  formation  of  a 
slight  precipitate.  If  we  continue  to  add  the  decimal  silver 
solution  until  the  turbidity  ceases,  and  once  more  add 
decimal  salt  solution,  we  shall  again  observe  the  formation 
of  another  precipitate.  If  we  determine  the  number  of  drops 
required  to  pass  from  one  limit  to  the  other,  we  observe  that 
the  same  number  of  each  is  needed.  Suppose  that  we  had 
added  in  the  first  place  the  salt  solution  until  the  exact  point 
was  reached  at  which  no  further  turbidity  was  produced,  and 
that  we  required  to  add  20  drops  of  the  silver  solution  before 
the  precipitate  again  ceased  to  form,  we  should  find  that  it 
would  be  necessary  to  add  20  drops  of  the  salt  solution  be- 
fore this  point  of  the  non-formation  of  a  turbidity  was  again 
reached.  If  we  add  exactly  half  this  number  of  drops,  viz., 
10,  we  reach  what  Mulder  terms  the  critical  point,  that  is,  the 
point  at  which  both  salt  and  silver  produce  equal  pre- 
cipitates. 

We  have,  therefore,  three  different  methods  of  determining 
the  completion  of  the  reaction  :  a.  We  may  add  the  salt 
solution  until  the  turbidity  just  ceases  ;  /3.  We  may  stop  at 
the  neutral  point ;  or  y.  We  may  go  back  with  silver  solution, 
and  continue  the  addition  until  no  further  turbidity  is  pro- 
duced. Whichever  method  we  adopt  in  standardising  the 
solution  of  salt  we  must  afterwards  invariably  employ.  Thus 
we  must  not  at  one  time  end  with  salt  and  at  another  end 
with  silver.  Mulder  has  shown  that  the  error  by  such  a 
procedure  amounts  to  i  milligram  per  gram  of  silver :  by 
employing  first  a  and  then  /3  the  difference  amounts  to  0-5 
milligram  at  16°  C.  Mulder  has  also  shown  that  the  degree 
of  error  varies  slightly  with  the  temperature  and  dilution  of 
the  liquids. 


Silver  Assay.  283 

On  the  whole  it  is  most  convenient  to  adopt  the  first  plan — 
i.e.  to  continue  the  addition  of  the  salt  until  no  further  pre- 
cipitate is  formed.  We  require  for  this  method  : 

1.  Solution  of  Sodium  Chloride. — Dissolve  5*4145  grams  of 
salt  in  distilled  water,  and  dilute  to  i  litre.  The  temperature 
of  the  solution  should  be   16°  when  measured.     100  c.c. 
of  this  solution  ==  i  gram  of  silver.    Call  it  *  Salt  Solution 
No.  i.' 

2.  Decimal  Solution  of  Sodium  Chloride. — Transfer  50  c.c. 
of  the  above  solution  to  a  J-litre  flask,  and  dilute  to  the  con- 
taining-mark  with  water  at  16°.    Call  it  '  Salt  Solution  No.  2.' 

3.  Decimal  Silver  Nitrate  Solution. — Weigh   out  exactly 
o'5  gram  of  pure  silver  (see  p.   124)  into  a  J-litre  flask,  dis- 
solve in  3  c.c.  of  pure  nitric  acid,  and  dilute  with  water  at 
1 6°  to  the  containing-mark.     i  c.c.  contains  i  milligram  of 
silver. 

4.  Test  Bottles. — The  bottles  specially  made  for  the  method 
should  be  procured.    They  are  of  white  glass,  about  250  c,c. 
in  capacity,  and  are  fitted  with  accurately- ground  stoppers, 
the  lower  portion  of  which  is  pointed.     On  touching  the 
side  of  the  bottle  with  the  point  of  the  moistened  stopper,  the 
adhering  liquid  is  readily  detached.     The  bottles  are  placed 
in  well-fitting  cases  of  cardboard  or  vulcanite,  and  when  in 
use  are  wrapped  in  a  black  cloth  in  order  to  protect  the  silver 
chloride  from  the  light. 

We  commence  the  process  by  determining  the  exact  value 
of  the  salt  solution.  Weigh  out  with  the  greatest  possible 
accuracy  from  i-ooi  to  1-003  gram  of  pure  silver,  place  it  in 
a  test  bottle,  add  5  c.c.  of  pure  nitric  acid  (sp.  gr.  1-2),  and 
heat  the  bottle  (placed  obliquely)  on  the  water-bath  until  the 
silver  is  dissolved.  From  time  to  time  blow  out  the  nitrous 
fumes  from  the  bottle,  occasionally  shaking  the  liquid  to 
promote  their  expulsion.  When  solution  is  effected,  allow 
the  bottle  to  cool  for  a  short  time  and  place  it  in  water  at  a 
temperature  of  about  16°.  Remove  it  in  about  half  an  hour, 
wipe  it,  and  place  it  in  its  case.  Transfer  100  c.c.  of  salt 


284  Quantitative  Chemical  Analysis. 

solution  No.  i,  measured  with  the  greatest  care,  to  the  bottle. 
Moisten  the  glass  stopper  with  distilled  water,  insert  it  firmly 
in  the  neck,  cover  the  bottle  with  the  black  cloth,  and  shake 
the  whole  violently  for  a  minute  or  two,  or  until  the  silver 
chloride  settles  completely,  leaving  the  fluid  perfectly  clear. 
Take  out  the  stopper,  rub  it  on  the  bottle  to  remove  the 
adherent  silver  chloride,  replace  it,  and  shake  down  the 
silver  chloride  on  the  sides  of  the  glass  by  giving  the  liquid 
a  rotatory  motion.  As  soon  as  the  chloride  is  deposited, 
again  open  the  bottle,  incline  it,  and  allow  \  c.c.  of  decimal 
salt  solution  (Sol.  No.  2)  to  flow  in  against  the  lower  part  of 
the  neck  of  the  bottle.  The  salt  solution  should  be  added 
from  a  Mohr's  burette,  graduated  into  OT  c.c.,  and  fitted 
with  a  glass  stopcock.  After  the  addition  of  the  i  c.c.  of  salt 
solution,  raise  the  bottle  from  its  case,  and  note  the  degree 
of  turbidity,  insert  the  stopper,  and  shake  until  the  liquid 
is  again  quite  clear.  Repeat  the  agitation  after  each  addition 
of  the  salt  solution,  and,  as  the  turbidity  decreases,  add  the 
solution  in  very  small  quantities  :  towards  the  end  only  two 
drops  should  be  added  at  a  time.  At  this  point  read  off  the 
burette,  and  continue  the  addition  of  the  salt  solution  by  two 
drops  at  a  time,  reading  off  the  burette  after  each  addition, 
and  agitating  the  liquid  until  no  further  precipitate  is  pro^ 
duced.  When  the  last  two  drops  fail  to  produce  a  turbidity 
the  process  is  at  an  end.  The  previous  reading — that  is,  the 
one  before  the  addition  of  the  last  two  drops — is  taken  as  the 
correct  one. 

If  by  mischance  the  exact  point  of  the  non-  formation  of  a 
turbidity  has  been  overstepped,  add  2  c.c.  of  decimal  silver 
solution,  shake,  and  continue  the  addition  of  the  salt  solution 
until  the  proper  point  is  again  reached. 

To  take  the  most  complicated  case  :  let  us  suppose  that 
we  have  weighed  off  1*0023  gram  of  pure  silver,  added 
100  c.c.  of  salt  solution  No.  i,  and  4*2  c.c.  of  the  decimal  salt 
solution.  We  have  reason  to  believe  that  we  have  over- 
stepped the  proper  point,  and  we  therefore  add  2  c.c.  of 


Silver  Assay.  285 

decimal  silver  solution,  and  again  1*8  c.c.  of  decimal  salt, 
when  turbidity  ceases. 

Amount  of  silver  taken  =  1-0023  +  -0020  =   1-0043  gram. 

This  required  100  c.c.  of  No.  i  salt  solution  +  6  c.c.  of 
No.  2  salt  solution,  or  altogether  100-6  c.c.  of  No.  i 
solution  for  complete  precipitation.  Calculate  the  quantity 
required  for  i  gram  of  silver  : 

1-0043    "     1*000     ••      100*6     =     100-17. 

We  thus  find  that  100*17  c.c.  of  the  salt  solution  No.  i  ex- 
actly precipitates  i  gram  of  pure  silver.  It  is  desirable  from 
time  to  time  to  repeat  the  determination  of  the  strength  of 
the  salt  solution. 

For  the  actual  assay,  weigh  out  about  i  -085  gram  of  standard 
silver  ( 1 2*3  of  silver  to  i  of  copper)  in  the  test  bottle,  dissolve 
in  5  or  6  c.c.  of  nitric  acid,  add  100  c.c.  of  the  salt  solution 
No.  i,  and  proceed  with  the  addition  of  the  decimal  salt 
solution  as  directed.  Let  us  assume  that  we  had  weighed 
out  1*085  gram>  and  tnat  it  was  necessary  to  add  6  c.c.  of 
decimal  silver  solution  before  turbidity  ceased  :  this,  with- 
out sensible  error,  we  may  assume  to  be  equal  to  0*6  of 
solution  No.  i.  We  thus  find  that  100*6  c.c.  of  the  strong 
solution  were  needed  to  precipitate  all  the  silver  in  the  1*085 
gram  of  the  alloy.  But  100*17  c-c-  of  the  solution  were 
equal  to  i  gram  of  pure  silver.  Therefore 

100*17   :   roo'6    ::    i    =    1*0043; 

and  accordingly  1,000  parts  of  the  alloy  would  contain 
1*085    •    i°°o    II     1*0043    =    925*6  parts  of  pure  silver. 

In  the  case  of  alloys  of  which  the  composition  is  not 
approximately  known  it  is  necessary  to  determine  it  by  a 
preliminary  trial  before  the  regular  assay  is  made.  Weigh 
off  from  0*5  to  i  gram  of  the  alloy  according  to  its  richness 
in  silver,  dissolve  in  4  or  5  c.c.  of  nitric  acid  in  the  usual 
manner,  and  add  sodium  chloride  solution  No.  i  from  a 


286  Quantitative  Chemical  Analysis. 

burette,  until  no  further  precipitate  is  formed.  Then  calculate 
the  amount  of  the  alloy  which  will  contain  1*002  gram  of 
silver,  and  proceed  with  the  assay  in  the  manner  directed. 

In  the  case  of  alloys  containing  sulphur  and  gold,  digest 
with  the  least  possible  quantity  of  nitric  acid,  blow  out  the 
nitrous  fumes  from  the  bottle,  add  strong  sulphuric  acid,  and 
boil  until  the  gold  separates  completely,  and  proceed  with 
the  assay  in  the  usual  way. 

XXXVI.     ASSAY  OF  GOLD. 

For  a  description  of  the  most  accurate  methods  of  assaying 
alloys  of  gold  by  cupellation,  we  would  refer  the  student  to 
Professor  Jevons'  excellent  article  on  the  subject  in  Watts' 
'  Dictionary  of  Chemistry,'  vol.  ii.  p.  932.  Longmans  :  1869. 

XXXVII.    SEPARATION  OF  GOLD,  SILVER,  AND  COPPER. 

I.  When  the  amount  of  silver  in  the  alloy  does  not  exceed 
15  per  cent,  the  whole  of  the  gold  and  copper  maybe  dissolved 
out  by  means  of  nitro-hydrochloric  acid,  the  silver  remaining 
as  silver  chloride.     The  solution  of  the  finely-divided  alloy 
after  complete  decomposition  is  evaporated  nearly  to  dryness 
to  expel  the  greater  portion  of  the  free  acid,  water  is  added, 
the  solution  filtered,  and  the  silver  chloride  is  washed,  dried, 
and  weighed.     To  the  solution  oxalic  acid  is  added,  whereby 
the  gold  is  completely  reduced  to  the  metallic  state.     After 
standing  for  about  48  hours,  the  liquid  is  filtered,  and  the 
gold  washed,  dried,  and  weighed.     The  copper  in  solution 
is  precipitated  as  sulphide  by  means  of  sulphuretted  hydro- 
gen :  it  may  either  be  weighed  as  such,  after  covering  it  with 
sulphur  and  heating  in  a  stream  of  sulphuretted  hydrogen, 
or  be  converted  into  oxide  by  re-solution  and  precipitation 
with  sodium  hydrate. 

II.  If,  on  the  other  hand,  the  amount  of  gold  in  the  alloy 
does  not  exceed  15  per  cent.,  the  whole  of  the  silver  is  dis- 
solved by  prolonged  boiling  with  moderately-concentrated 


Mercury.  287 

nitric  acid.*  The  liquid  is  evaporated  nearly  to  dryness, 
water  added,  the  silver  precipitated  by  hydrochloric  acid, 
and  the  copper  in  the  filtrate  by  sulphuretted  hydrogen. 
The  gold  after  weighing  is  dissolved  in  cold  nitro-hydro- 
chloric  acid,  to  ascertain  that  it  is  perfectly  free  from  silver. 
If  any  silver  chloride  is  found  it  must  of  course  be  filtered 
off  and  weighed. 

III.  By  heating  alloys  of  gold,  silver,  and  copper  with 
strong  sulphuric  acid,  the  metals  may  be  separated,  whatever 
may  be  their  proportion.  The  finely-divided  alloy  is  heated 
with  the  concentrated  acid  until  no  more  sulphur  dioxide  is 
evolved,  and  the  acid  begins  to  volatilise.  Water  is  then 
added  and  the  liquid  boiled,  and  the  silver  and  copper 
separated  as  above.  It  is  advisable  to  treat  the  gold  again 
with  the  acid  before  finally  weighing  it :  if  any  additional 
silver  and  copper  are  obtained  their  weight  must  of  course 
be  added  to  the  main  quantities  of  these  metals. 

XXXVIII.  ESTIMATION  OF  MERCURY. 
In  ores  and  compounds  containing  mercury  the  amount 
of  the  metal  may  be  readily  determined  by  heating  the  sub- 
stance with  quicklime.  The  process  is  conducted  in  the 
apparatus  represented  in  fig.  65.  Into  a  piece  of  combus- 
tion tube,  about  50  centimetres  long  and  rounded  at  the 
end,  introduce  a  layer,  5  centimetres  in  length,  of  powdered 
magnesite  (a).  Weigh  out  about  5  grams  of  the  substance 
into  a  glazed  porcelain  mortar  and  mix  it  intimately  with 
quicklime.  Introduce  the  mixture  into  the  tube  (a  to  b\ 
and  rinse  out  the  mortar  with  a  fresh  portion  of  quicklime 
(b  to  c) ;  fill  up  the  rest  of  the  tube  to  within  a  few  centi- 
metres of  the  end  with  powdered  quicklime  (c  to  d)t  and  draw 
out  the  tube  before  the  blow-pipe  in  the  manner  represented 
in  the  cut.  Gently  tap  the  tube  so  as  to  make  a  channel  for 

*  The  nitric  acid  must  not  be  too  strong,  otherwise  more  than  traces 
of  the  gold  would  be  dissolved  by  the  nitrous  acid  formed. 


288  Quantitative  Chemical  Analysis. 

the  gas,  and  place  it  in  the  combustion  furnace,  and  immerse 
the  drawn-out  end  just  beneath  the  surface  of  water  contained 
in  a  small  flask.  Heat  the  tube  from  d  to  <r,  at  first  gently, 
and  then  to  bright  redness,  and  gradually  heat  the  portion  of 


FIG.  65. 
t>        c 


the  tube  containing  the  mercurial  compound  to  redness. 
When  the  substance  is  completely  decomposed,  heat  the 
magnesite  to  expel  the  traces  of  mercurial  vapour  within  the 
tube.  Whilst  the  tube  is  still  red  hot,  cut  off  the  tube  at  *, 
and  wash  any  adhering  mercury  into  the  flask.  Agitate  the 
mercury  beneath  the  surface  of  the  water  so  as  to  bring  it 
together  into  one  globule,  and  in  about  half  an  hour  decant  the 
clear  water ;  pour  the  mercury  on  to  a  small  weighed  watch- 
glass,  remove  the  water  as  far  as  practicable  by  filter-paper, 
and  before  re-weighing  it  place  the  watch-glass  and  metal 
for  an  hour  or  two  beneath  a  bell-jar  containing  strong  sul- 
phuric acid.  The  only  mercurial  compound  which  thus 
resists  complete  decomposition  is  the  iodide  ;  it  may  be 
readily  analysed,  however,  by  substituting  copper  filings  for 
the  lime.  Cinnabar  may  be  readily  analysed  by  heating  it 
with  nitric  acid  of  sp.  gr.  i  -4  in  a  closed  tube  for  a  couple 
of  hours  :  the  sulphur  is  converted  into  sulphuric  acid,  which 
may  be  determined  as  barium  sulphate.  Silica,  heavy  spar, 
&c.,  remain  undissolved.  The  mercury  is  precipitated  as 
calomel  by  phosphorous  acid — as  described  below. 

In  the  case  of  very  poor  ores  of  cinnabar  containing  large 


Coal.  289 

quantities  of  bituminous  matter,  such  as  those  of  Austria, 
the  foregoing  process  may  be  judiciously  modified  by  first 
extracting  the  organic  matter  from  the  powdered  ore  by  means 
of  benzol,  thoroughly  drying  the  residue,  and  treating  it  with 
a  solution  of  barium  sulphide  (containing  about  50  grams  of 
barium  to  the  litre).  The  mercuric  sulphide  is  precipitated 
from  the  solution  by  means  of  hydrochloric  acid,  filtered, 
dried,  and  digested  with  carbon  bisulphide  to  remove  ad- 
mixed sulphur.  The  impure  mercuric  sulphide  is  then 
dried,  re-dissolved  in  aqua  regia,  the  liquid  concentrated, 
and  mixed  with  solution  of  phosphorous  acid,  allowed  to 
stand  about  12  hours  in  the  cold,  and  the  precipitated  calo- 
mel collected  on  a  weighed  filter,  washed  with  hot  water, 
and  dried  at  100°. 

Mercury  may  be  readily  determined  by  electrolysis.  The 
solution  of  the  metal  is  poured  into  a  weighed  platinum  dish 
and  slightly  acidified  with  sulphuric  acid,  and  the  dish  is 
connected  with  the  zinc  pole  of  a  battery  of  six  bichromate 
cells,  the  carbon  end  being  attached  to  a  piece  of  platinum 
foil  which  dips  into  the  solution.  Mercurous  chloride 
gradually  separates,  and  this  is  finally  reduced  to  the  metal, 
and  in  about  an  hour  all  the  mercury  is  precipitated.  The 
metal  is  washed  with  water,  alcohol,  and  ether,  and  dried 
over  oil  of  vitriol  and  weighed. 

[Note  on  the  Preparation  of  Phosphorous  Acid. — Small  pieces  of 
phosphorus  are  introduced  into  a  saturated  solution  of  copper  sulphate 
contained  in  a  corked  flask ;  metallic  copper  is  first  reduced,  which  is 
ultimately  converted  into  black  copper  phosphide,  and  the  acid  solution 
consists  of  a  mixture  of  phosphorous  and  sulphuric  acids.  The  latter 
may  be  removed  by  the  cautious  addition  of  baryta  water.  The  pre- 
cipitate is  allowed  to  settle,  and  the  clear  solution  of  phosphorous  acid 
decanted  and  preserved  in  a  well-stoppered  bottle,  as  it  oxidises  on 
exposure  to  air.  It  is  a  very  powerful  reducing  agent,  and  may  be  use- 
fully applied  in  a  variety  of  cases.] 

XXXIX.     COAL. 

In  the  proximate  analysis  of  coal  we  require  to  determine 
the  amount  of  moisture,  volatile  matter,  coke,  ash,  and 

u 


290  Quantitative  Chemical  A  nalysis. 

sulphur.  If  the  actual  amount  of  carbon,  hydrogen,  and 
nitrogen  is  needed,  recourse  must  be  had  to  elementary 
organic  analysis. 

Determination  of  the  Moisture. — About  2  grams  of  the 
finely-powdered  coal  are  weighed  out  and  dried  between 
watch-glasses  at  105-110°  for  an  hour,  and  the  loss  is  set 
down  as  moisture.  With  the  quantity  of  coal  taken  the  loss 
of  weight  appears  to  be  greatest  at  the  end  of  this  time :  on 
further  heating,  it  actually  increases  in  weight,  an  effect  due 
probably  to  the  oxidation  of  the  finely- divided  pyrites. 

Determination  of  the  Volatile  Matter. — About  2  grams  of 
the  powdered  undried  coal  are  heated  for  four  minutes  over 
a  Bunsen  flame,  and  then  immediately,  without  cooling,  for 
the  same  length  of  time  over  the  gas  blow-pipe  flame.  The 
loss  is  set  down  as  volatile  matter  +  moisture.  The  residue 
gives  the  coke + ash. 

Determination  of  the  Ash. — From  3  to  5  grams  of  the 
finely- divided  coal  are  heated  in  a  small  platinum  dish  over 
a  Bunsen  lamp.  Usually  the  incineration  proceeds  with 
rapidity :  if  it  is  found  necessary  to  increase  the  draught  of 
air  over  the  heated  mass,  the  arrangement  described  in  the 
section  on  Analysis  of  Ashes  of  Plants  may  be  employed. 

Determination  of  the  Stilphur. — The  sulphur  in  coal  exists 
in  two  modifications — as  pyrites  and  as  calcium  sulphate. 
The  sulphur  contained  in  the  pyrites  alone  influences  the 
economical  application  of  the  fuel. 

The  total  amount  of  sulphur  may  be  determined  by  heat- 
ing about  2  grams  of  the  powdered  coal  with  four  times  its 
weight  of  pure  sodium  carbonate  in  a  platinum  dish,  or  the 
coal  may  be  heated  in  a  current  of  oxygen  and  the  gases 
passed  through  a  solution  of  hydrochloric  acid  and  bromine, 
the  sulphuric  acid  being  determined  by  precipitation  with 
barium  chloride.  The  sulphur  existing  as  calcium  sulphate 
may  be  determined  by  boiling  5  grams  of  the  finely-powdered 
coal  with  a  solution  containing  about  the  same  weight  of  pure 
sodium  carbonate,  whereby  the  calcium  sulphate  is  decom- 


Water  Analysis.  291 

posed,  calcium  carbonate  and  sodium  sulphate  being  formed. 
Filter  the  solution,  acidify  with  hydrochloric  acid,  and  add 
barium  chloride.  The  difference  between  the  total  amount 
of  sulphur  and  that  found  after  boiling  with  sodium  car- 
bonate represents  the  amount  as  pyrites.  The  same  process 
is  of  course  applicable  to  the  determination  of  the  iron 
sulphide  and  gypsum  in  coke. 

Determination  of  Specific  Gravity. — It  is  occasionally 
desirable  to  ascertain  the  weight  of  a  cubic  foot  of  the  fuel, 
or  the  number  of  cubic  feet  corresponding  to  a  ton.  This  is 
easily  calculated  when  the  specific  gravity  of  the  coal  is 
known.  The  specific  gravity  is  readily  determined  by  weigh- 
ing the  coal  in  air ;  and  in  water  by  suspending  it  from  the 
arm  of  the  balance  by  a  hair  or  thin  wire.  The  piece  taken 
should  not  be  too  small,  and  care  should  be  taken  that  no 
air  bubbles  adhere  to  it  during  the  weighing.  It  is  desirable, 
too,  that  the  coal  be  soaked  sufficiently  :  this  is  easily  effected 
by  immersing  the  lump,  after  attaching  the  hair  or  wire  to 
it,  in  water,  in  the  flask  of  the  filter-pump,  and  exhausting 
the  air  within  the  apparatus  as  far  as  practicable.  The 
weight  of  a  cubic  foot  of  the  coal  in  pounds  is  found  from 
the  expression : 

log.  sp.  gr.  +  i'79588=log.  wt.  of  cb.  foot. 
The  number  of  cubic  feet  in  a  ton 

=•=1  '55437  -log-  sp.  gr.=log.  cb.  ft. 

XL.    EXAMINATION  OF  WATER  USED  FOR  ECONOMIC 
AND  TECHNICAL  PURPOSES. 

i.  Collection  of  the  Sample. — The  water  to  be  analysed 
should  be  collected  in  stoppered  glass  bottles — those  known 
as  ' Winchester  Quarts,'  which  hold  about  2  J  litres,  may  be 
conveniently  employed.  As  a  rule  two  of  these  bottles  will 
contain  sufficient  water  for  an  ordinary  examination.  If, 
however,  an  exhaustive  analysis  is  required,  double  or  even 
treble  this  amount  may  be  necessary.  Care  must  be  taken 

u  2 


292  Quantitative  Chemical  Analysis. 

that  the  bottles,  and  the  vessels  employed  to  fill  them,  are 
quite  clean,  and  a  due  amount  of  judgment  should  be  used  to 
obtain  a  representative  sample.  In  collecting  the  water  from 
a  river  or  tank  the  bottles  themselves  should  be  immersed  be- 
low the  surface,  and  rinsed  once  or  twice  with  the  water.  In 
taking  the  water  from  a  pump  or  pipe  a  considerable  quan- 
tity should  be  allowed  to  flow  away  before  the  sample  is 
collected.  The  bottles  should  be  filled  up  nearly  to  the  neck 
and  the  stopper  tied  down  with  string :  no  luting  or  sealing- 
wax  should  be  used. 

Glass  bottles  are  preferable  to  stoneware  jars,  for  the 
reason  that  earthenware  is  not  readily  cleaned  ;  moreover  it  is 
liable  to  affect  the  hardness  of  the  water,  as  the  clay  not  un- 
frequently  contains  notable  quantities  of  calcium  sulphate. 

2.  Preliminary  Observations. — Fill  a  tall  narrow  cylinder 
of  white  glass  with  the  water  to  be  examined,  and  compare 
its  colour  with  that  of  distilled  water  contained  in  a  similar 
cylinder.  Heat  a  portion  to  about  30°  in  a  wide  test-tube, 
shake,  and  note  if  the  water  possesses  any  peculiar  odour  or 
taste. 

In  the  outset  the  analyst  must  decide  whether  the  water  for 
analysis  is  to  be  filtered  or  not.  His  decision  will  depend  upon 
the  manner  in  which  the  water  is  used  by  the  consumer.  If  it 
be  considered  necessary  to  filter  the  sample,  care  must  be 
taken  that  the  paper  employed  is  free  from  ammonia.  It 
should  be  steeped  in  distilled  water  for  some  time  before 
use,  dried  and  folded,  and  heated  in  a  weighed  tube 
for  some  hours  at  120°.  It  is  placed  in  the  desiccator 
and  weighed  when  perfectly  cold.  It  is  now  properly  fitted 
into  the  funnel,  and  the  filter-flask  is  replaced  by  a  clean 
*  Winchester  Quart,'  in  which  the  filtrate  is  directly  received. 
The  quantity  of  the  water  to  be  filtered  is  measured  ;  as  soon 
as  the  whole  has  passed  through,  the  funnel  is  removed  from 
the  bottle  and  washed  with  distilled  water  (the  washings 
must  not  be  allowed  to  mix  with  the  filtered  water),  again 


Water  Analysis.  293 

dried  at  120°  for  some  hours  in  the  stoppered  tube,  and  again 
weighed.  The  increase  in  the  weight  gives  the  quantity  of 
total  suspended  matter  in  the  known  volume  of  the  water. 
Incinerate  the  paper  in  a  small  platinum  crucible,  treat  the 
residue  with  a  few  drops  of  solution  of  ammonium  carbonate, 
dry  and  weigh ;  the  quantity  in  excess  of  that  contained  in  the 
filter  gives  the  amount  of  suspended  inorganic  matter  in  the 
water. 

If  it  is  considered  unnecessary  to  filter  the  water,  care 
should  be  taken  to  shake  the  bottle  before  withdrawing 
portions  for  analysis. 

3.  Estimation  of  the  Ammonia. — It  is  desirable  to  proceed 
at  once  with  the  determination  of  this  constituent,  since  it  is 
the  most  liable  to  change.  The  method  of  estimation  is 
based  upon  the  fact  that  an  alkaline  solution  of  mercuric 
iodide  added  to  a  liquid  containing  ammonia  produces  a 
brown  colouration,  due  to  the  formation  of  the  iodide  of 
tetramercurammonium.  This  test,  known  as  Nessler's,  is 
capable  of  detecting  i  part  of  ammonia  in  20,000,000  parts 
of  water. 

Preparation  of  the  Nessler  Test.— Dissolve  35  grms.  of 
potassium  iodide  in  120  c.c.  of  water,  transfer  5  c.c.  of  the 
solution  to  a  clean  beaker,  and  add,  little  by  little,  a  cold  con- 
centrated solution  of  mercuric  chloride  to  the  remainder 
until  the  mercuric  iodide  ceases  to  be  re-dissolved  on  stirring. 
Add  the  5  c.c.  of  the  potassium  iodide  to  re-dissolve  the 
remaining  mercuric  iodide,  and  cautiously  continue  the  ad- 
dition of  the  corrosive  sublimate  solution  until  a  very  slight 
precipitate  only  remains.  Now  add  an  aqueous  solution  of 
potash,  prepared  by  dissolving  100  grams  of  'stick5  potash 
in  200  c.c.  of  water,  and  dilute  the  mixture  to  500  c.c.  The 
liquid  should  be  allowed  to  stand  for  a  short  time,  and  a 
portion  decanted  into  a  small  bottle  for  use.  As  the  small 
bottle  becomes  empty  it  is  replenished  from  the  other.  In 
addition  we  require : 


294  Quantitative  Chemical  Analysis. 

(a)  A  Standard  Solution  of  Ammonium  Chloride. — Dis- 
solve 07867  grm.  of  pure  ammonium  chloride  in  a  litre  of  dis- 
tilled water.  Pour  it  into  a  clean  stoppered  bottle,  and  label 
it  c  Ammonium  Chloride  Solution  No.  i.'  Withdraw  100 
c.c.  of  this  solution  and  dilute  it  also  to  i  litre.  Call  it 
'Ammonium  Chloride  Solution  No.  2.'  i  c.c.  of  the  latter 
solution  contains  -025  of  a  milligram  of  ammonia.  The  solu- 
tion should  be  delivered  from  a  Mohr's  burette,  fitted  with 
glass  stopcock,  and  graduated  to  tenths  of  a  cubic  centi- 
metre. 

(/3)  A  small  Pipette  to  deliver  about  i  c.c.  of  the  Nessler 
Test. — This  may  readily  be  made  from  a  short  piece  of  glass 
tube.  Also  several  cylinders,  marked  A,  B,  c,  D,  &c., 
about  20  cm.  in  height  and  of  60  c.c.  capacity.  To  graduate 
them,  transfer  50  c.c.  of  water  to  each,  and  mark  the  level  of 
the  liquid  on  the  glass.  Also  two  or  three  pieces  of  thin  glass 
tube,  about  30  cm.  in  length,  and  3  mm.  in  external  diameter, 
the  ends  of  which  should  be  blown  into  bulbs  of  such 
diameter  that  they  will  readily  pass  into  the  cylinders  ;  the 
other  ends  are  sealed. 

(y)  Distilled  Water  free  from  Ammonia. — The  distilled 
water  of  the  laboratory  must  be  tested  for  ammonia.  Rinse 
one  of  the  cylinders  with  the  water  and  fill  it  up  nearly 
to  the  top ;  add  i  c.c.  of  the  clear  Nessler  solution,  and 
agitate  with  the  bulb-tube  (/3)  by  drawing  it  up  and  down  a 
few  times  within  the  cylinder.  If,  after  standing  for  five 
minutes,  the  water  remains  perfectly  uncoloured,  it  may  be 
considered  free  from  ammonia.  If  it  shows  a  yellow  or  brown 
tint,  re-distil  it  after  addition  of  about  i  gram  of  pure  sodium 
carbonate ;  collect  the  distillate  in  a  Winchester  quart,  as 
soon  as  50  c.c.  received  in  one  of  the  cylinders  gives  no 
reaction  for  ammonia  on  testing  with  the  Nessler  solution. 
If  ordinary  water  is  used,  the  distillation  must  not  be  carried 
to  dryness,  and  the  water  remaining  in  the  retort  or  boiler 
must  be  thrown  away  before  a  fresh  quantity  is  distilled. 


Water  Analysis.  295 

TJee  Process. — Transfer  50  c.c,  of  the  natural  water  to  be 
tested  to  one  of  the  glass  cylinders  standing  on  a  sheet  of 
white  paper,  add  i  c.c.  of  the  Nessler  solution  and  agitate 
with  the  bulb-tube.  Run  50  c.c.  of  distilled  water  into  a 
second  cylinder,  add  *2  c.c.  of  Ammonium  Chloride  Solution 
No.  2,  mix  thoroughly,  and  compare  the  tints  in  the  two 
cylinders.  If  they  are  about  equal  in  intensity,  take  half  a 
litre  for  the  estimation :  if  the  coloration  in  the  natural 
water  is  the  more  intense,  take  a  proportionately  less  quantity. 
This  testing  is  simply  preliminary  :  its  object  is  to  afford  an 
idea  of  the  proper  quantity  to  take  for  the  actual  estimation. 
Observe  whether  the  natural  water  becomes  turbid  after  the 
addition  of  the  Nessler  test :  a  decided  precipitate  is  due  to 
lime  -or  magnesia  salts,  and  indicates  hardness.  500  c.c.  of 
the  water,  or  a  less  quantity  if  the  preliminary  testing  has 
shown  that  ammonia  is  present  in  considerable  amount,  are 
placed  in  a  capacious  retort,  and  connected  with  a  Liebig's 
condenser,  which  should  be  freed  from  ammonia  by  previously 
blowing  steam  through  it  for  a  few  minutes.  If  less  than  500 
c.c.  of  water  have  been  taken,  the  liquid  in  the  retort  should  be 
made  up  to  this  volume  by  the  addition  of  pure  distilled 
water  before  the  distillation  is  commenced.  Add  about  i 
gram  of  recently-heated  and  pure  sodium  carbonate,  note  if 
much  precipitate  is  formed,  and  distil  rapidly  over  the  direct 
flame.  Collect  50  c.c.  of  the  distillate  in  one  of  the  cylinders, 
A  ;  when  filled  replace  it  by  a  second  cylinder,  B.  When 
the  second  cylinder  is  full,  remove  the  lamp  ;  add  i  c.c.  of 
Nessler's  solution  to  the  second  50  c.c.  of  the  distillate  (i.e.  B), 
agitate,  and  place  it  on  a  sheet  of  white  paper.  Now  fill  up 
a  third  cylinder,  z,  with  pure  distilled  water  to  within  a  few 
centimetres  of  the  level  of  that  in  B,  add  as  much  standard 
Ammonium  Chloride  Solution  No.  2  as  you  think  will  pro- 
duce the  same  depth  of  colour  as  in  B,  and  afterwards  i  c.c. 
of  Nessler's  solution  :  add  a  little  distilled  water  if  necessary, 
so  as  to  make  the  two  levels  in  the  tubes  coincident. 
Agitate  and  compare  the  tints.  If  the  colour  of  the  liquid  in 


296  Quantitative  Chemical  Analysis. 

the  two  cylinders,  after  standing  about  5  minutes,  is  equal, 
we  at  once  know  the  amount  of  ammonia  contained  in  B  : 
it  is  equal  to  that  contained  in  the  volume  of  standard  am- 
monium chloride  solution  added  to  z.  If  the  intensity  in  z  is 
not  equal  to  that  in  B,  pour  away  the  contents  of  the  former 
cylinder,  rinse  it,  fill  it  with  a  second  portion  of  distilled 
water,  add  more  or  less  ammonium  chloride  solution,  as  the 
case  may  be,  and  i  c.c.  of  the  Nessler  test.* 

If  the  quantity  of  ammonia  in  B  does  not  exceed  -01  of  a 
milligram  (equal  to  0-4  c.c.  of  the  standard  ammonium 
chloride  solution),  the  distillation  may  be  discontinued  :  if 
the  amount  is  greater  than  this,  the  boiling  must  be  renewed, 
and  successive  portions  of  50  c.c.  of  the  distillate  tested  until 
the  above  limit  is  reached.  If  the  quantity  of  ammonia  in  B 
does  not  exceed  that  corresponding  to  0-8  c.c.  of  the 
standard  solution  of  ammonium  chloride,  the  amount  in  A 
may  be  determined  in  the  manner  directed  ;  if  the  quantity  is 
greater  than  this,  25  c.c.,  or  less  if  need  be,  of  the  solution  must 
be  transferred  to  another  cylinder,  diluted  to  50  c.c.  with 
pure  distilled  water  and  tested  as  above.  A  colour  produced 
by  more  than  4  c.c.  of  the  ammonium  chloride  solution  can- 
not be  conveniently  compared,  since  the  liquid  is  apt  to 
become  turbid.  In  the  case  of  waters  known  to  contain 
much  ammonia,  as  in  sewage,  distil  over  100  c.c.  at  once 
into  a  larger  cylinder,  withdraw  an  aliquot  portion,  dilute  to 
50  c.c.,  and  titrate  in  the  manner  directed. 

The  determination  of  the  ammonia  in  water  used  for 
drinking  is  of  great  importance,  since  an  undue  proportion 
of  this  substance  denotes  contamination  with  sewage. 
Sewage  may  contain  from  2  to  10  parts  of  ammonia  in 
100,000  parts  of  liquid  :  river- waters  maybe  said  to  contain 
on  the  average  about  o-oi  part,  although  this  amount  is 


*  The  addition  of  more  ammonium  chloride  solution  after  the  Nessler 
test  has  been  mixed  with  the  liquid  would  cause  a  turbidity,  which  pre- 
vents accurate  comparison, 


Water  Analysis.  297 

subject  to  great  variation.     Bad  well-waters  sometimes  con- 
tain as  much  as  0-5  to  i  part  in  100,000  parts. 

Estimation  of  '  Albuminoid  Ammonia? — Messrs.  Wanklyn 
and  Chapman  have  found  that  many  nitrogenous  organic 
substances  yield  either  the  whole  or  a  definite  portion  of 
their  nitrogen  in  the  form  of  ammonia  when  boiled  with  an 
alkaline  solution  of  potassium  permanganate.  Hippuric 
acid  parts  with  all  its  nitrogen  as  ammonia  when  thus  treated  : 
whereas  albumen  gives  up  only  10  per  cent,  of  ammonia, 
uric  acid  7  per  cent,  and  creatine  12-6  per  cent.  Since  it  is 
highly  probable  that  the  azotised  organic  matter  contained 
in  water  is  of  an  albuminoid  nature,  its  quantity  may  be 
approximately  estimated  by  determining  the  quantity  of 
ammonia  yielded  by  boiling  the  water  with  an  alkaline  solu- 
tion of  potassium  permanganate ;  according  to  Messrs. 
Wanklyn  and  Chapman,  'the  disintegrating  animal  refuse' 
in  the  water  'would  be  pretty  fairly  measured  by  ten  times  the 
albuminoid  ammonia  which  it  yields.'  * 

The  liquid  remaining  in  the  retort  after  distillation  with 
sodium  carbonate  (Estimation  of  Ammonia)  is  mixed  with  50 
c.c.  of  a  solution  obtained  by  dissolving  8  grams  of  potassium 
permanganate  and  200  grams  of  potassium  hydrate  in  i  litre 
of  distilled  water.  The  mixture  should  be  boiled  for  some 
time  previous  to  use  and  preserved  in  a  well-stoppered 
bottle.  After  the  addition  of  the  permanganate  solution, 
heat  the  retort  over  the  naked  flame,  and  distil  successive 
portions  of  50  c.c.,  and  determine  the  quantity  of  ammonia 
present  in  them  with  the  Nessler  solution.  As  soon  as  the 
distillate  contains  less  than  '01  milligram  of  ammonia,  the 
process  may  be  considered  at  an  end.  Add  together  the 
several  quantities  of  ammonia  obtained.  The  succussive 
ebullitions  occasionally  noticed  in  bad  water  may  be  dimin- 
ished by  throwing  a  number  of  recently-ignited  pieces  of 
pumice  into  the  liquid. 

*  Wanklyn  and  Chapman,  '  Water  Analysis,'  p.  68. 


298  Quantitative  Chemical  Analysis. 

Estimation  of  Organic  Carbon  and  Nitrogen  (Frankland 
and  Armstrongs  Process). 

Drs.  Frankland  and  Armstrong  have  proposed  to  estimate 
the  carbon  and  nitrogen  contained  in  the  organic  matter 
present  in  water  by  direct  combustion.  The  water  is 
evaporated  to  dryness,  and  the  residue  is  mixed  with  cupric 
oxide  and  burnt  as  in  the  elementary  analysis  of  an  organic 
compound.  The  resultant  gas  is  collected  over  mercury, 
and  the  proportion  of  carbon  dioxide  and  nitrogen  deter- 
mined by  gasometric  analysis.  The  process  occupies  con- 
siderable time,  but  if  the  evaporation  of  the  water  be 
commenced  as  soon  as  the  ammonia-determination  has  been 
made,  the  residue  will  be  ready  for  the  combustion  (which 
occupies  about  an  hour)  by  the  time  that  the  hardness,  total 
soluble  matter,  nitrates,  &c.,  have  been  estimated. 

i.  Evaporation  of  the  Water. — The  quantity  of  the  water 
needed  for  analysis  will  depend  upon  its  quality..  If  less  than 
0*05  part  of  ammonia  in  100,000  parts  of  water  has  been 
found,  a  litre  should  be  taken ;  if  more  than  0-05,  but  less 
than  0-2,  half  a  litre  will  suffice;  if  more  than  0-2,  and  less 
than  i  -o,  a  quarter  of  a  litre  should  be  used.  Of  sewage,  which 
is  much  richer  in  organic  carbon  and  nitrogen,  100  c.c.,  or 
even  less,  may  be  taken. 

Transfer  the  measured  quantity  of  the  water  to  a  large 
flask,  add  to  it  20  c.c.  of  a  saturated  solution  of  washed 
sulphurous  acid,  and,  if  it  does  not  exceed  250  c.c.,  boil  the 
mixture  for  a  few  seconds  to  expel  the  carbon  dioxide  pre- 
sent. Transfer  the  water,  little  by  little,  to  a  hemispherical 
glass  dish,  10  centimetres  in  diameter,  and  shaped  somewhat 
like  a  finger-bowl :  during  the  evaporation  the  glass  dish 
should  be  supported  in  a  copper  basin  provided  with  a  pro- 
jecting flange  and  resting  on  the  water-bath,  and  over  it 
should  be  placed  a  glass  shade,  about  1 2  in.  high  (such  as 
is  used  for  covering  statuettes).  The  steam  condenses  in 
the  inside  of  the  shade  and  flows  down  into  the  copper  dish, 
filling  the  space  between  the  two  dishes.  The  excess  of 


Water  Analysis.  299 

water  flows  out  by  a  small  lip  on  the  edge  of  the  copper  dish, 
and  is  led  off  by  a  piece  of  tape.  The  destruction  of  the 
nitrates  and  nitrites  by  the  sulphur  dioxide  may  be  greatly 
accelerated  by  the  addition  of  2  or  3  drops  of  ferrous  chloride 
solution  (prepared  by  dissolving  well-washed  ferrous  hydrate 
precipitated  from  ferrous  sulphate  by  pure  soda  solution  in 
the  minimum  quantity  of  pure  hydrochloric  acid)  to  the  first 
dishful  of  the  water  \  if  the  water  is  free  from  carbonates  it 
will  be  necessary  also  to  add  i  or  2  c.c.  of  a  solution  of 
sodium  bisulphite  in  order  to  combine  with  the  sulphuric  acid 
formed,  which  if  free  would  decompose  the  organic  matter  on 
concentration.  If,  however,  the  water  in  the  glass  dish  or 
flask  ceases  at  any  time  during  the  progress  of  the  evapora- 
tion to  smell  of  sulphur  dioxide,  more  of  the  solution  should 
be  added.  If  the  water  is  found  to  contain  much  nitric 
acid  it  may  be  necessary  to  digest  the  residue  with  ,a  dilute 
solution  of  sulphur  dioxide,  and  again  evaporate  to  dry- 
ness,  to  ensure  the  complete  elimination  of  the  inorganic 
nitrogen. 

2.  Combustion  of  the  Residue. — Introduce  a  small  quantity 
of  cupric  oxide  in  fine  powder  (made  by  oxidising  the  metal 
in  air*)  into  the  dish,  and  mix  it  thoroughly  with  the  residue 
by  the  aid  of  a  small  steel  spatula  :  this  should  be  very 
elastic,  so  that  by  accommodating  itself  to  the  curvature  of 
the  dish  the  dried  residue  may  be  completely  detached  from 
the  glass.  Fill  about  3  centimetres  of  the  carefully-cleaned 
combustion  tube  (which  should  be  about  40  centimetres  in 
length  and  i  centimetre  in  internal  diameter,  sealed  at 
one  end  and  rounded  like  a  test-tube)  with  pure  copper 
oxide,  and  transfer  the  whole  of  the  mixture  in  the  glass  dish 
to  the  tube,  rinsing  the  dish  with  small  successive  portions  of 

*  The  copper  oxide  may  be  prepared  by  cutting  copper  wire  or  sheet 
into  small  pieces,  washing  the  metal,  and  heating  it  in  a  muffle.  The  oxide 
obtained  by  strongly  heating  the  copper  nitrate  cannot  be  well  employed, 
as  it  is  very  difficult  to  expel  the  last  traces  of  nitrogen  from  it.  The 
cupric  oxide  remaining  in  the  tube  after  the  combustion  (with  the  excep- 
tion of  that  with  which  the  substance  was  mixed)  may  be  used  again 
after  it  has  been  re-heated  in  a  current  of  air. 


30O  Quantitative  Chemical  Analysis. 


FIG.  66.' 


£ 


7 


FIG.  67. 


Water  A  nalysis.  30 1 

pure  cupric  oxide  in  fine  powder.  Add  copper  oxide  to  the 
tube  until  it  is  a  little  more  than  half-filled,  insert  a  cylinder 
of  metallic  copper,  about  8  centimetres  in  length,  made  by 
wrapping  fine  copper  gauze  round  a  piece  of  thick  copper 
wire,*  and  then  add  2  centimetres  of  granular  copper 
oxide  in  order  to  oxidise  any  carbon  monoxide  which 
might  be  formed  on  burning.  The  end  of  the  tube  is 
softened  in  the  flame  of  the  blow-pipe  and  drawn  out  to 
form  a  tube  about  150  millimetres  long  and  4  millimetres  in 
diameter.  Bend  the  tube  at  right  angles,  fuse  the  edges  in 
the  flame,  place  it  in  the  combustion  furnace  and  attach  it 
to  the  Sprengel  pump. 

Fig.  66  shows  the  arrangement  of  this  apparatus  as  applied 
to  the  purpose  of  exhausting  the  combustion  tube,  a  is  a 
glass  funnel  maintained  full  of  mercury,  and  connected  by 
means  of  a  short  piece  of  caoutchouc  tube,  on  which  is  a 
screw  clamp,  />,  with  a  long  narrow  tube  which  passes  nearly  to 
the  bottom  of  a  wider  tube,  d,  90  centimetres  in  length  and 
about  i  centimetre  in  internal  diameter  ;  the  upper  end  of 
d  is  connected  with  a  glass  funnel  in  the  manner  represented 
in  the  figure,  d  is  connected  with  the  tube/^  by  a  piece 
of  strong  caoutchouc  tube  covered  with  tape  and  provided 
with  a  screw  clamp.  The  tube  fg  is  about  6  millimetres  in 
diameter  and  600  millimetres  in  length;  it  is  attached  to  a 
tube,gVz  k,  about  1,500  millimetres  long  and  6  millimetres  in 
external  diameter,  but  with  a  bore  of  only  i  millimetre.  The 
portion  g  h  is  about  20  centimetres  long ;  the  portion  h  k  is 
about  130  centimetres.  To  give  them  stability  the  tubes  are 
fastened  together  by  caoutchouc  and  copper  wire.  At  the 
upper  portion  of  the  bend  is  a  tube,  h  /,  about  1 2  centimetres 
long  and  5  millimetres  in  diameter.  The  combustion  tube 

*  The  cylinder  must  be  previously  heated  in  the  lamp,  so  as  to 
oxidise  it  superficially  :  it  is  then  placed  in  a  tube  and  heated  in  a 
current  of  hydrogen,  in  order  to  reduce  the  oxide  formed.  The 
cylinder  must  be  allowed  to  cool  in  the  gas  before  it  is  withdrawn  from 
the  tube. 


302  Quantitative  Chemical  Analysis. 

o  is  connected  with  the  tube  h  I  by  the  tube  /  m  n, 
of  the  same  diameter  as  the  tube  h  k.  The  tube  /  m  n  is 
connected  with  the  tube  h  I  and  with  the  combustion  tube 
by  short  lengths  of  well-fitting  caoutchouc  tube ;  the  joint 
at  /  is  bound  round  with  copper  wire,  and  is  surrounded 
with  glycerine,  contained  in  the  wider  tube  supported  by  a 
cork  on  h  I;  the  joint  at  n  is  in  like  manner  surrounded  by 
a  wider  tube  filled  with  water.  On  the  tube  lmn'\<$>  a  small 
bulb,  which  is  immersed  in  cold  water  during  the  combus- 
tion ;  its  object  is  to  receive  the  greater  portion  of  the  water 
formed  on  burning  the  residue.  The  tube  h  k  is  re-curved  at 
k,  where  it  ends  in  the  mercury-trough  /.  The  trough 
p  (shown  in  plan  at  B,  fig.  67)  is  cut  out  of  a  solid 
piece  of  mahogany.  It  is  20  centimetres  long,  15-5  centi- 
metres wide,  and  10  centimetres  deep,  outside  measurement. 
The  edge  rr  is  13  millimetres  wide,  and  the  shelf  s  is  65 
millimetres  wide,  174  millimetres  long,  and  50  millimetres 
deep  from  the  top  of  the  trough.  The  channel  /  is  25  milli- 
metres wide,  and  75  millimetres  deep  ;  at  one  end  of  it  is  a 
circular  well,  w,  42  millimetres  in  diameter,  and  90  milli- 
metres deep.  The  recesses  u  u  receive  the  re-curved  ends 
of  the  Sprengel  pumps  :  the  object  of  having  two  recesses  is 
to  allow  of  two  experiments  being  made  simultaneously  : 
each  recess  is  40  millimetres  long,  25  millimetres  wide,  and 
75  millimetres  deep.  The  tubes  destined  to  receive  the  gases 
are  supported  against  the  iron  wires  v  v.  The  trough  stands 
upon  four  short  legs,  and  has  a  side  tube  and  clamp,  ^,  to 
draw  off  the  mercury  to  the  level  of  the  shelf  s  when 
necessary. 

When  everything  is  arranged,  heat  the  fore  part  of  the 
tube  containing  the  metallic  copper  and  unmixed  copper 
oxide,  and  allow  a  gentle  stream  of  mercury  to  flow  from 
the  funnel  a  :  on  reaching  h  the  metal  passes  down  the  tube 
h  k  in  detached  portions,  each  carrying  before  it  a  small 
quantity  of  air  from  the  combustion  tube.  Care  must  be 
taken  so  to  control  the  flow  of  mercury  that  it  does  not  rise 
into  the  tube  /  m  n.  The  bulb  on  the  tube  I  m  n  is  sur- 


Water  Analysis. 

FIG.  6a 


303 


304  Quantitative  Chemical  Analysis. 

rounded  by  hot  water  during  the  exhaustion  in  order  to 
expel  any  moisture  which  may  remain  in  it  from  a  previous 
experiment.  If  the  fall  is  properly  regulated  the  exhaustion 
will  be  complete  in  about  ten  minutes,  when  the  mercury 
will  be  heard  to  fall  with  a  sharp  clicking  sound.  The 
action  of  the  pump  is  now  arrested  ;  a  small  tube  filled  with 
mercury  is  inverted  over  the  end,  k,  of  the  tube,  the 
hot  water  in  x  is  replaced  by  cold  water,  and  the  rest  of  the 
tube  is  gradually  heated  to  redness.  In  about  an  hour  the 
combustion  will  be  terminated.  The  pump  is  again  set  in 
operation  and  the  gases  are  transferred  to  the  tube. 

Measurement  and  Analysis  of  the  Gases. — The  gases  pro- 
duced in  the  combustion  consist  of  carbon  dioxide,  nitrogen, 
nitrogen  dioxide,  and  occasionally,  if  the  operation  has  been 
conducted  too  rapidly,  sulphur  dioxide  and  carbon  mon- 
oxide. Their  measurement  and  analysis  may  be  conveni- 
ently made  in  the  apparatus  seen  in  fig.  68,  which  is  essen- 
tially that  devised  by  Frankland  for  the  separation  of  gases 
incidental  to  water- analysis,  a  c  d  is  the  measuring  tube  : 
the  portion  a  is  about  370  millimetres  long  and  18  milli- 
metres wide,  c  is  40  millimetres  long  and  7  millimetres 
wide,  and  d  is  175  millimetres  long  and  2-5  millimetres 
in  diameter.  To  the  upper  end  of  d  is  attached  a  tube 
with  capillary  bore,  bent  at  right  angles  and  provided 
with  a  stopcock,  /.  The  measuring  tube  is  graduated  from 
below  upwards  at  intervals  of  10  millimetres,  the  zero  being 
about  100  millimetres  from  the  lower  end.  The  upper  por- 
tion of  d  is  divided  into  millimetres.  Attached  to  the  tube 
and  stopcock/,  is  a  steel  cap,  shown  on  a  larger  scale  at  B, 
fig.  68.  The  lower  portion  of  a  is  drawn  out  until  it  is  only 
about  5  millimetres  wide  :  the  tube  b,  which  is  about  i  -2 
metre  long  and  6  millimetres  in  internal  diameter,  is  also 
narrowed  at  the  lower  end.  Both  a  and  b  pass  through  the 

*  In  newer  forms  of  the  apparatus  Frankland  has  dispensed  with  the 
steel  caps  :  the  tube  from  the  laboratory  vessel  being  fitted  by  a  cap  of 
un vulcanised  caoutchouc  into  a  cup- shaped  vessel  attached  to  the  ca- 
pillary tube  of  the  measuring  apparatus. 


Water  Analysis. 


305 


caoutchouc  stopper  o,  which  is  fitted  into  the  glass  cylinder 
n  n,  which  is  filled  with  cold  water  with  the  view  of  giving  a 
definite  temperature  to  the  enclosed  gas  :  this  temperature 
is  ascertained  by  a  thermometer,  e,  suspended  by  a  hook 
from  the  edge  of  the  cylinder.  Uniformity  in  the  tempera- 
ture of  the  mass  of  the  water  may  be  ensured  by  agitating  it 
with  an  iron  wire,  the  end  of  which  is  bent  in  the  form  of  a 
ring.  The  tube  b  is  graduated  into  millimetres,  the  zero 
being  about  10  millimetres  above  the  stopper  o,  and  on  a 
level  with  that  of  a  c  d.  The  rubes  a  and  b  are  supported 
by  the  wooden  clamp  p  (seen  in  end  elevation  and  plan  at 
B  and  c);  the  clamp  is  drawn  together  by  two  screws,  the 
tubes  being  covered  with  caoutchouc  where  they  fit  into  the 
holes  to  protect  them  from  breakage.  The  clamp  is  sup- 
ported by  an  upright  piece  of  wood  (seen  in  B)  which  is 
screwed  into  the  base :  a  and  b  are  connected  by  tubes 
of  caoutchouc,  covered  with  tape  and  bound  with  wire,  to 
the  tube  ^,  which  is  also  connected  with  the  long  caout- 
chouc tube  leading  to  the  glass  reservoir  /.  This  tube, 
which  should  have  an  internal  diameter  of  not  more  than  2 
millimetres,  passes  through  the  steel  clamp  r,  the  lower  por- 
tion of  which  is  fixed  into  /.  The  reservoir  t  is  suspended 
by  a  cord  passing  over  pulleys,  in  the  arm  of  the  iron  rod  s. 
On  releasing  the  loop  on  the  cord  from  the  hook  v,  the  re- 
servoir sinks  from  about  10  centimetres  above  the  level  of 
the  stopcock  /  to  the  level  of  the  bottom  of  the  clamp  /. 
In  the  jar  >£,  termed  the  laboratory  vessel,  the  gases  are 

FIG.  69. 

2>  &      E      fl  ' 


subjected  to  the  action  of  absorbents.     It  is  100  millimetres 
high  and  40  millimetres  in  internal  diameter,  and  is  fur- 

x 


306  Quantitative  Chemical  Analysis. 

nished  with  a  capillary  tube,  glass  stopcock,  and  steel  cap, 
g  h,  exactly  like  fg.  The  mercury  trough  /,  seen  in  plan 
in  D  and  in  section  in  E  in  fig.  69,  is  cut  out  of  a  solid  piece  of 
mahogany :  it  is  265  millimetres  long,  80  millimetres  broad, 
and  90  millimetres  deep,  outside  measurement.  The  rim 
a  a  is  8  millimetres  broad  and  15  millimetres  deep.  The 
channel  b  is  230  millimetres  long,  26  millimetres  broad, 
and  65  millimetres  deep.  In  the  larger  excavation  at  the 
end  of  the  channel  is  placed  the  laboratory  vessel :  it  is  45 
millimetres  in  diameter  and  20  millimetres  in  depth  below 
the  top  of  b.  The  small  cavities  c  c  are  to  receive  the 
capsule  employed  to  transfer  the  tube  containing  the  gases 
from  the  trough  of  the  Sprengel  pump.  The  trough  /  rests 
on  a  telescope-table,  and  its  height  is  so  adjusted  that  when 
the  laboratory  vessel  is  placed  in  the  cavity  the  faces  of  the 
steel  caps  are  in  exact  coincidence.  (Fig.  68.) 

Before  using  the  instrument  the  '  corrections  for  capillarity ' 
must  be  determined.  When  the  mercury  in  the  tube  b  and 
in  the  measuring  tube  a  c  d  is  freely  exposed  to  atmospheric 
pressure,  it  will  be  noticed  that  the  levels  of  the  metal  in  the 
two  tubes  are  not  coincident ;  the  level  in  b  is  slightly  higher 
than  in  a ;  on  the  other  hand  the  level  in  c  and  d  will  be 
found  to  be  higher  than  that  in  b.  The  difference  in  each 
portion  should  be  determined  by  taking  several  observations  : 
the  correction  will  also  include  the  error  arising  from  dif- 
ference of  level  in  the  zeros  of  the  graduations  of  b  and  a  c  d. 
The  determination  of  the  levels  should  be  made  by  the  aid 
of  a  telescope  sliding  on  a  vertical  rod. 

To  determine  the  capacity  of  the  measuring  tube  at  each 
graduation,  first  fill  the  entire  tube  with  mercury,  so  that  the 
metal  drips  from  the  cap  g.  Close  the  stopcock/,  and  slip 
a  piece  of  caoutchouc  tube  over  the  cap  :  attach  the  other 
end  of  the  tube  to  a  funnel  filled  with  distilled  water ;  lower 
the  reservoir  /,  and  open  the  clamp  r  and  the  stopcock/. 
As  the  mercury  flows  into  the  reservoir,  water  is  drawn  through 


Water  Analysis.  307 

the  capillary  tube.  As  soon  as  it  is  below  the  zero  on  a,  close 
/,  remove  the  caoutchouc  tube  from  the  cap  and  slightly  grease 
it,  to  allow  water  to  pass  through  it  without  adhering.  Raise 
the  reservoir,  open/,  and  expel  the  water  until  the  upper  por- 
tion of  the  mercury  meniscus  is  coincident  with  the  zero  of  the 
graduation.  Now  allow  the  water  to  flow  out  into  a  small  tared 
flask  until  the  level  of  the  mercury  is  coincident  with  the 
next  graduation,  controlling  the  influx  of  the  mercury  by 
the  clamp  r.  Read  off  the  temperature  of  the  water  in  the 
cylinder  «,  weigh  the  water  in  the  flask,  and  calculate  its 
volume  from  Table  II.  in  the  Appendix.  Repeat  the  deter- 
mination between  successive  graduations  on  the  whole  length 
of  the  tube  in  exactly  the  same  manner. 

A  table,  showing  the  capacity  of  the  tube  at  various  points, 
is  then  constructed,  the  intermediate  graduations  being  ob- 
tained by  interpolation  :  as  the  calculations  are  much  facili- 
tated by  the  use  of  logarithms,  it  will  be  found  more  con- 
venient to  set  down  in  this  table  the  logarithms  of  the  capa- 
cities in  place  of  the  natural  numbers. 

To  use  the  apparatus,  grease  the  stopcocks/ and  h  and 
the  faces  of  the  caps  g  and  g'  with  a  little  resin  cerate  mixed 
with  oil.  Fill  a  c  d  with  mercury  and  close  /.  Place  the 
laboratory  vessel  in  its  cavity,  and  suck  out  the  air  as  far  as 
practicable  by  the  aid  of  a  caoutchouc  tube,  which  is  re- 
moved as  soon  as  the  jar  is  filled.  Any  remaining  air  may  be 
drawn  away  by  aspirating  at  g'.  Close  h,  and  fasten  the 
faces  of  the  caps  tightly  together  by  the  aid  of  the  clamp  A. 
Of  course  the  entire  apparatus  must  be  quite  free  from  air, 
FlG-  70.  and  on  opening  the  stopcocks  the 

mercury  should  flow  freely  through 
the  capillary  tubes.  To  determine 
if  the  several  joints  of  the  apparatus 
are  air-tight,  close  h,  and  lower  the 
reservoir,  until  it  is  on  a  level  with^. 
Since  the  stopcocks  and  joinings  are 
thus  .subjected  to  a  pressure  of  nearly  half  an  atmo* 

X  2 


308  Quantitative  Chemical  Analysis. 

sphere,  any  imperfection  which  may  cause  leakage  will  be 
readily  detected.  After  the  trial  replace  t  in  its  original 
position. 

The  gas  to  be  analysed  is  decanted  into  the  laboratory 
vessel  and  treated  with  one  or  two  drops  of  strong  potassium 
bichromate  solution,  to  ascertain  if  it  is  free  from  sulphur 
dioxide.  If  this  gas  is  absent  the  colour  of  the  solution  will 
be  unaltered;  if  present  a  portion  of  the  chromic  trioxide 
will  be  reduced,  and  the  liquid  will  become  green.  If  any 
change  is  observed,  pass  up  a  few  more  drops  of  the  solution, 
to  complete  the  absorption  of  the  gas.  Open  the  stopcocks 
and  lower  the  reservoir,  and  transfer  the  gas  to  the  measuring 
tube  ;  close  h  so  soon  as  the  liquid  in  the  laboratory  vessel 
is  within  10  mm.  from  the  stopcock.  The  quantity  of  gas 
remaining  in  the  capillary  tube  is  too  minute  to  affect  the 
experiment.  The  apex  of  the  mercury  meniscus  in  a  c  d  (as 
seen  through  the  telescope)  is  made  to  coincide  with  the 
nearest  division  on  the  tube  by  allowing  mercury  to  flow  in 
from  the  reservoir  /.  Read  off  the  levels  of  the  mercury  in 
tubes  b  and  a  c  d ;  note  the  temperature  of  the  water  in  n, 
together  with  the  height  of  the  barometer. 

Pass  a  few  drops  of  strong  potash  solution  by  means  of  a 
pipette  into  the  laboratory  vessel,  and  return  the  gas  to  it. 
The  absorption  of  the  carbon  dioxide  will  be  complete  in 
about  five  minutes.  The  gas  now  consists  of  nitrogen  mixed 
with  a  small  quantity  of  nitric  oxide  ;  it  is  again  brought  into 
the  measuring  tube,  and  its  volume  is  ascertained  in  the 
same  manner  as  before.  If  the  volume  of  the  gas  is  very 
small  it  is  possible  that  it  may  already  contain  oxygen  ;  if  so, 
any  nitric  oxide  which  might  have  been  formed  will  have  been 
converted  into  nitrogen  tetroxide,  which  will  have  been  ab- 
sorbed by  the  potash  solution.  The  volume  of  gas  abstracted 
in  this  case  is,  however,  too  small  to  affect  the  result.  To 
ascertain  if  oxygen  is  present,  pass  up  a  small  quantity  of  a 
cold  saturated  aqueous  solution  of  pyrogallic  acid  into  the 
jar,  and  by  gently  shaking  the  stand  of  the  trough,  throw  the 


Water  Analysis.  309 

liquid  up  against  the  sides  of  the  jar  in  order  to  promote 
the  absorption.     As  soon  as  the  liquid  runs  down  from  the 

glass  without  the  forma- 

FlG.    71. 

tion  of  a  dark  red  stain, 
the  absorption  of  the 
oxygen  is  complete.  If 
oxygen  be  absent,  it  will 
be  necessary  to  intro- 
duce a  few  bubbles  of 
that  gas  in  order  to  ox- 
idise the  nitrogen  dioxide  which  may  be  mixed  with  the 
nitrogen.  The  addition  of  the  oxygen  may  be  conveniently 
made  from  the  pipette  shown  in  fig.  71.  The  bulbs  #and  b 
are  about  5  cm.  in  diameter ;  the  neck  joining  them  is 
narrowed,  so  that  mercury  flows  through  it  but  slowly.  To 
use  the  instrument  fill  the  bulb  b  and  the  tubes  d  and  c 
with  mercury  ;  introduce  the  tube  ^into  a  small  tube  contain- 
ing oxygen,  standing  over  the  mercury  trough,  and  gently 
aspirate  by  the  aid  of  the  caoutchouc  tube  e.  A  few  bubbles 
are  readily  drawn  over  into  b,  and  the  gas  is  confined  by  the 
mercury  in  d  and  c  \  on  introducing  the  limb  d  beneath  the 
edge  of  the  laboratory  jar  and  gently  blowing  through  ^,  the 
oxygen  may  be  transferred  to  the  gas  under  examination. 
Allow  the  mixed  gases  to  remain  in  contact  with  the  potash 
solution  for  a  few  minutes.  When  the  nitrogen  tetroxide  and 
excess  of  oxygen  have  been  absorbed,  transfer  the  gas  back 
again  to  the  measuring  tube  and  determine  the  volume  of 
the  residual  nitrogen. 

The  three  reduced  volume-readings — ist,  of  the  total 
gas  (A)  ;  2nd,  of  the  nitrogen  and  nitrogen  dioxide  (B)  ;  and 
3rd,  of  the  nitrogen  (c) — furnish  all  the  data  for  obtaining  the 
total  volume  of  nitrogen  and  carbon  dioxide  in  the  gaseous 
mixture. 

A    —    B       =       VOl.  Of  CO2. 

B  +  c     =     vol.  of  N. 


Quantitative  Chemical  Analysis. 


From  the  corrected  volumes  the  weights  of  the  carbon 
and  nitrogen  are  readily  calculated. 

The  calculation,  as  Dr.  Frankland  has  pointed  out,  may 
be  simplified  by  considering  the  original  gaseous  mixture  as 
nitrogen,  so  far  as  volume-weight  is  concerned.  If  A  be  the 
weight  of  the  total  gas,  B  its  weight  after  treatment  with 
potash,  and  c  after  absorption  by  pyrogallate,  the  weight 
of  carbon  will  be  f  (A— B),  and  the  weight  of  nitrogen 

?  +  c?    since  the  weights  of  carbon  and  nitrogen  in  equal 

volumes  of  carbon  dioxide  and  nitrogen,  measured  under 
the  same  conditions,  are  as  6  :  14,  and  the  weights  of 
nitrogen  in  equal  volumes  of  that  gas  and  of  nitrogen  dioxide 
are  as  2  \  i.  By  using  the  annexed  logarithmic  table  for  the 
reduction  of  cubic  centimetres  of  nitrogen  to  grams  for  each 
tenth  of  a  degree  centigrade,  the  calculation  becomes  the 
work  of  a  few  moments  only. 

Table  for  the  reduction  of  Cubic  Centimetres  of  Nitrogen  to  Grams. 


Log 


0-0012562 
(i  +  0-00367/)  760 


for  each  tenth  of  a  degree  from  o°  to  30°  C. 


t° 

o'o 

0*1 

O'2 

°'3 

0-4 

o'S 

0-6 

0*7 

0-8 

0-9 

0° 

6-21824 

808 

793 

777 

761 

745 

729 

713 

697 

681 

I 

665 

649 

633 

617 

601 

586 

570 

554 

538 

522 

2 

507 

491 

475 

429 

443 

427 

412 

396 

380 

364 

3 

349 

333 

3i8 

302 

286 

270 

255 

239 

223 

208 

4 

192 

177 

161 

H5 

130 

114 

098 

083 

067 

051 

5 

035 

020 

004 

*989 

*973 

*957 

*942 

*926 

*9» 

*895 

6 

6-20879 

864 

848 

833 

817 

801 

786 

770 

755 

739 

7 

723 

708 

692 

676 

661 

645 

629 

614 

598 

8 

567 

552 

536 

52i 

505 

490 

474 

459 

443 

428 

9 

413 

397 

382 

3^6 

35  i 

335 

320 

3°4 

289 

274 

10 

259 

244 

228 

213 

198 

182 

167 

151 

136 

121 

ii 

106 

090 

075 

060 

045 

029 

014 

*999 

*984 

*969 

12 

6-19953 

938 

923 

907 

892 

877 

862 

846 

831 

816 

13 

800 

785 

770 

755 

740 

724 

709 

694 

679 

664 

H 

648 

633 

618 

603 

588 

573 

558 

543 

528 

513 

15 

497 

482 

467 

452 

437 

422 

407 

392 

377 

362 

Water  A  nalysis.  311 

Table  for  the  reduction  of  Cubic  Centimetres  of  Nitrogen  to  Grams — cont. 


t° 

o'o 

O'l 

0'2 

0-3 

°'4 

o'S 

o'6 

0-7 

0.8 

o-9 

16 

346 

331 

3^ 

301 

286 

271 

256 

241 

226 

211 

17 

196 

181 

166 

J57 

136 

121 

106 

091 

076 

06  1 

18 

046 

031 

016 

001 

*986 

*97I 

*956 

*94i 

*926 

*9ii 

19 

6*18897 

882 

867 

852 

837 

822 

807 

792 

777 

762 

20 

748 

733 

718 

703 

688 

673 

659 

644 

629 

614 

21 

600 

58S 

570 

555 

540 

526 

5ii 

496 

481 

466 

22 

452 

437 

422 

408 

393 

378 

363 

349 

334 

319 

23 

24 

305 

158 

290 
143 

% 

261 
114 

246 
099 

084 

216 
070 

202 

°55 

187 
041 

172 
026 

25 

OI2 

*997 

*982 

*968 

*953 

*938 

*924 

*909 

*895 

*88o 

26 

6-I7866 

851 

837 

822 

808 

793 

779 

764 

750 

735 

27 

721 

706 

692 

677 

663 

648 

634 

619 

605 

590 

28 

576 

561 

547 

532 

518 

503 

489 

475 

460 

446 

29 

432 

417 

403 

388 

374 

360 

345 

331 

316 

302 

An  example  of  the  mode  of  calculation  will  serve  to  render 
the  process  more  intelligible.  Let  us  assume  that  we  have 
made  the  following  readings  : — 

A.  B.  c. 


Volume  of  gas 
Temperature 


(After  treatment 
with  KHO) 

5  -oo  c.  c.  0-40  c.  c.     0-40  c.  c. 

15°  I5-I0  I5-20 


Height  of  mercury  in  a,  c,  d .     300 
i,  ,i  b  .          . 

Difference     .... 
Add  tension  of  aqueous  vapour 
(Table  III.  Appendix) 


Deduct  for  capillarity     . 

Deduct  from  height  of  bar 
Pressure  on  dry  gas 


mm.                           mm. 
300                          480 
200                            350 

loo                    130 
127                   12-8 

mm. 
480 
330 

150 

12-9 

162-9 

112-7                142-8 
Add  for    f 
0  capillarity  \- 

111-7                 J45'3 

760-0                 760-0 
in-7                 145-3 

165-4 

760-0 
165-4 

6483 


614-7          594-6 


312  Quantitative  Chemical  Analysis. 

Log.  of  vol.  of  gas   .."*,."   0-69897      1-60206   1-60206 

5"9482 


Pressure  on  dry  gas        .         .         2-81178  2-78866      2-77422 

Log.  of  weight  calc.  as  N.     .        37O572  4'58554      4*57095 

Weight  calculated  as  N.       -0050783  -00038507     '00037235 

Weight  of  carbon  =  3(  -0050783  --0003851)  =  O<O02O1I4 


Weight  of  nitrogen  =  .O003787I 

Sometimes,  especially  when  the  amount  of  carbon  is 
large,  small  quantities  of  carbon  monoxide  may  be  formed, 
and  may  escape  complete  oxidation  by  the  copper  oxide 
placed  in  the  anterior  portion  of  the  tube.  This  gas  remains 
mixed  with  the  nitrogen  after  absorption  with  potassium 
pyrogallate  solution.  Its  amount  may  be  determined  when 
the  whole  of  the  gas  is  transferred  to  the  measuring  tube,  in 
the  last  determination  of  volume,  by  removing  the  labora- 
tory vessel,  washing  it,  refilling  it  with  mercury,  and  again 
attaching  it  to  the  face  of  the  cap.  A  few  drops  of  solution 
of  cuprous  chloride  are  then  introduced  into  the  vessel,  and 
the  gas  allowed  to  act  upon  it.  In  about  five  minutes  the 
absorption  of  the  carbon  monoxide  will  be  complete  ;  the 
residual  nitrogen  may  then  be  returned  to  the  measuring 
tube,  and  its  volume  determined.  If  any  carbon  monoxide 
is  found,  its  weight  as  nitrogen  is  calculated  in  the  manner 
described,  and  added  to  that  corresponding  to  the  carbon 
dioxide  before  multiplying  by  fj  its  weight  must  also  be 
deducted  from  that  corresponding  to  the  volume  after  treat- 
ment with  potash. 

Since  the  accuracy  of  this  method  of  combustion  depends 
upon  the  perfection  of  the  vacuum  obtained  by  the  Sprengel 
pump,  and  is  liable  to  be  affected  to  some  slight  degree  by 
nitrogen  retained  in  the  copper  oxide,  absorption  of  ammo- 
nia during  the  evaporation,  &c.,  it  is  advisable  that  the 


Water  A  nalysis.  313 

experimenter  should  perform  several  blank  determinations, 
to  ascertain  the  accumulated  effect  of  these  errors.  This 
should  be  done  by  evaporating  to  dryness  in  the  manner 
described  a  litre  of  pure  distilled  water,  with  the  usual 
quantities  of  sulphurous  acid  and  ferrous  chloride  solutions, 
together  with  about  0*1  gram  of  recently  ignited  sodium 
chloride,  to  afford  a  residue  wrhich  can  be  transferred  to  the 
tube.  The  residue  is  then  to  be  burnt,  and  the  gases  analysed 
as  directed  :  the  amounts  of  carbon  and  nitrogen  thus  found 
are  to  be  deducted  from  the  quantities  obtained  in  the  sub- 
sequent analyses  of  water-residues.  The  corrections  should 
amount  to  '00006  gram  of  carbon,  and  '00005  °f  nitrogen 
per  litre  of  water.  The  amount  of  nitrogen  existing  as  NH3 
must  be  subtracted  from  the  quantity  of  N  thus  found ;  the 
remainder  may  be  set  down  as  organic  nitrogen. 

The  estimation  of  the  organic  carbon  and  nitrogen  in 
water  is  of  great  importance  in  determining  the  degree  of 
organic  contamination  which  it  has  experienced.  Good 
drinking  water  should  not  contain  more  than  0*2  part  of 
carbon,  and  0*02  part  of  nitrogen  per  100,000  parts  of  water. 
Sewage  usually  contains  about  four  parts  of  carbon,  and  two 
parts  of  nitrogen.  The  ratio  of  carbon  to  nitrogen  is  of 
especial  importance ;  the  lower  the  ratio  the  more  objection- 
able is  the  organic  matter.  The  ratio  in  water  for  domestic 
supply  may  vary  from  five  to  twelve ;  sewage  varies  from 
one  to  three  ;  polluted  river- water  from  three  to  five.  (For 
further  details  consult  Frankland's  '  Water  Analysis,'  Van 
Voorst,  1880.) 

Dittmarr's  and  Robinson's  process. — In  this  process  the 
organic  carbon  is  determined  by  weighing  as  carbon  dioxide 
as  in  an  ordinary  combustion,  whilst  the  organic  nitrogen  is 
converted  into  ammonia  by  ignition  with  soda  or  soda-baryta, 
as  in  the  method  given  on  p.  334. 

Determination  of  Organic  Carbon. — Evaporate  i  litre  of 
the  water,  after  treatment  with  i  c.c.  of  a  saturated  solution 


3 1 4  Quantitative  Chemical  A  nalysis. 

of  sulphurous  acid,  as  described  on  p.  298,  in  a  glass  dish 
under  the  bell  jar,  and  transfer  the  dried  residue  by  the 
aid  of  a  spatula  to  a  platinum  boat,  which  is  then  to 
be  introduced  into  a  short  combustion  tube,  one  end  of 
which  has  been  previously  drawn  out,  and  into  which  is 
placed  (i)  a  spiral  of  silver  wire  gauze  to  reduce  any  nitrogen 
oxides  which  may  be  formed,  and  (2)  a  layer  of  granular 
copper  oxide.  To  the  drawn-out  end  of  the  tube  is  attached 
a  small  V-shaped  tube,  containing  a  solution  of  chromic 
acid  in  60  per  cent,  sulphuric  acid,  to  which  is  adapted 
a  short  tube  filled  with  calcium  chloride.  The  carbon 
dioxide  is  absorbed  in  a  light  tube  containing  soda-lime 
and  calcium  chloride,  previously  weighed.  The  platinum 
boat  containing  the  residue  having  been  placed  in  the 
combustion  tube,  the  posterior  end  is  connected  by  a 
cork  and  bent  tube  with  a  gas-holder  containing  air  freed 
from  carbon  dioxidt.  The  oxide  of  copper  and  silver  are 
first  heated,  and  then  the  platinum  boat,  a  stream  of  the 
purified  air  being  meanwhile  sent  through  the  apparatus. 
The  carbon  dioxide  freed  from  water  and  sulphur  dioxide  by 
passing  through  the  chromic  acid  solution  is  absorbed  by 
the  soda-lime. 

Determination  of  Organic  Nitrogen. — Place  half  a  litre 
of  the  water  in  a  flask  connected  with  a  condenser,  and 
distil,  collecting  the  distillate  in  the  manner  directed  on 
p.  295,  so  as  to  determine  the  ammonia.  The  distillation 
is  to  be  conducted  until  only  about  30  c.c.  are  left  in  the 
flask.  Add  solutions  of  sulphurous  acid  and  ferrous  chloride 
to  destroy  nitrates,  and  if  the  solid  residue  is  known  to  be 
small,  add  too  a  small  quantity  of  potassium  sulphate. 
Complete  the  evaporation  under  the  bell  jar,  and  when  dry 
transfer  the  residue  to  a  large  silver  or  copper  boat,  moisten 
with  a  drop  of  water,  and  cover  with  2  or  3  grams  of  a  fused 
mixture  of  equal  weights  of  baryta  and  soda.  Introduce  the 
boat  into  a  short  combustion  tube  connected  at  one  end 
with  a  U-tube  containing  a  known  volume  of  water  acidu- 


Water  A  nalysis.  315 

lated  with  a  few  drops  of  pure  hydrochloric  acid,  and 
to  the  other  end  adapt  an  arrangement  for  sending  a 
current  of  hydrogen  through  the  apparatus.  Heat  the 
tube  to  redness;  the  organic  nitrogen  is  converted  into 
ammonia,  which  is  absorbed  by  the  dilute  hydrochloric 
acid :  its  amount  is  estimated  by  the  Nessler  test,  as  de- 
scribed on  p.  295,  on  one-tenth  of  the  solution  diluted  to 
50  c.c. 

Blank  experiments  should  be  made  in  both  determina- 
tions, and  the  necessary  corrections  introduced. 

Estimation  of  Total  Soluble  Matter. — Ignite  and  weigh  a 
platinum  dish,  place  it  on  a  glass  ring  on  the  water-bath,  and 
fill  it  with  the  water  to  be  examined,  previously  measured  in 
a  250  c.c.  flask.  As  the  liquid  evaporates,  add  successive 
portions  from  the  flask ;  rinse  the  vessel  when  empty  with  a 
small  quantity  of  distilled  water,  and  pour  the  washings  into 
the  dish.  When  the  water  has  entirely  evaporated,  heat  the 
residue  to  100°,  for  an  hour,  or  until  its  weight  is  constant. 
The  increase  in  the  weight  of  the  dish  gives  the  amount  of 
soluble  matter  contained  in  the  ^-litre  of  water. 

Estimation  of  Nitrates  and  Nitrites* — This  may  be 
effected  in  the  residue  obtained  from  the  preceding  deter- 
mination, by  the  action  of  precipitated  copper  and  zinc. 
The  apparatus  seen  in  fig.  72  serves  for  the  decomposition. 
A  is  a  flask  of  100  c.c.  capacity,  fitted  with  a  caoutchouc 
stopper,  containing  (i)  the  tube  funnel  b,  provided  with  a 
glass  stopcock,  and  (2)  the  bent  glass  tube  c,  which  is  con- 
nected, by  means  of  a  well-fitting  stopper,  with  one  of  the 
cylinders,  <?,  used  for  the  ammonia  estimation  (p.  292).  This 
stopper  also  carries  the  bent  tube  d,  which  is  about  5  milli- 
metres in  internal  diameter,  and  is  partially  filled  with  frag- 
ments of  well-washed  glass.  Place  3  or  4  grams  of  very 

*  Jour.  Chem.  Soc.  June  1873. 


3  £6 


Quantitative  Chemical  Analysis. 


FIG.  72. 


thin  sheet-zinc  in  small  pieces  in  A,  and  cover  it  with  a 
tolerably  concentrated  solution  of  copper  sulphate.  Allow 
the  solution  to  act  upon  the  zinc  for  ten  or  fifteen  minutes, 
pour  off  the  supernatant  liquid,  and  fill  up  the  flask  several 
times  with  cold  distilled  water  to  wash  the  precipitated  cop- 
per. After  the  last  washing,  remove  the  water  as  far  as 
practicable.  Add  about  25  c.c.  of  distilled  water  to  the 

residue  obtained  in  the 
determination  of  the 
total  soluble  matter,  to- 
gether with  a  piece  of 
recently  ignited  lime, 
about  the  size  of  a 
hemp-seed,  and  boil 
the  liquid  (to  destroy 
any  urea  which  may  be 
present),  until  4  or  5  c.c. 
only  remain.  Transfer 
the  liquid  to  the  flask  A, 
rinse  the  dish  with  dis- 
tilled water,  so  as  to 
make  up  the  volume  in 
A  to  about  15  c.c.  or 
20  c.c.  Fit  in  the  cork 
of  the  flask,  add  one 
drop  of  dilute  hydro- 
chloric acid  (free  from 
ammonium  chloride)  to 
the  cylinder  *,  together  with  2  or  3  c.c.  of  distilled  water,  and 
also  moisten  the  glass  in  d  with  two  drops  of  the  acid.  Fit 
the  cork  into  <?,  and  place  the  tube  in  a  beaker  of  cold  water, 
as  represented  in  fig.  72.  Heat  the  liquid  in  A  to  boiling, 
and  distil  it  over  into  e ;  when  A  is  nearly  empty,  fill  up  the 
funnel  with  hot  water,  turn  the  stopcock,  allow  the  water  to 
flow  into  the  flask,  and  continue  the  ebullition  until  e 
contains  about  40  c.c.  of  liquid.  All  the  nitrates  will  be 


Water  Analysis.  317 

reduced,  and  the  ammonia  will  be  expelled.  Raise  the 
retort  stand  so  as  simultaneously  to  remove  the  tube  from 
the  water  in  the  beaker,  and  the  flask  A  from  over  the  lamp. 
Wash  the  fragments  of  glass  in  d\  the  water  is  readily 
drawn  over  into  e  by  the  contraction  of  the  air  in  A  on 
cooling.  Fill  up  the  tube  e  to  the  mark,  and  agitate  the 
liquid  by  the  aid  of  the  bulb-stirrer.  Transfer  5  c.c.  to 
a  second  tube,  dilute  with  distilled  water,  add  i  c.c.  of 
Nessler's  solution,  and  agitate.  If  the  degree  of  colouration 
is  measurable,  determine  the  quantity  of  ammonia  required 
to  produce  it  in  the  manner  described  on  p.  295 ;  if  the  tint 
is  too  dark  for  comparison,  take  a  smaller  quantity;  if  too 
light  (as  it  will  be  unless  the  water  is  very  bad),  take  a 
larger  quantity,  say  10  c.c.  or  20  c.c.  of  the  distillate,  in 
accordance  with  the  indications  of  the  preliminary  trial. 
The  following  determinations  made  on  known  quantities  of 
nitre  may  serve  to  show  the  degree  of  accuracy  of  which 
this  method  is  capable : 

TAKEN.    FOUND.      TAKEN.    FOUND.       TAKEN.    FOUND. 
mgms.     mgms.       mgms.     mgms.         mgms.     mgms. 

1-67          I1'68  2-50.     ,(2'42  3-34-     •  3-22 

'  \i-72  \2'49  4'J6  .     .  4-01 

If  the  operator  possesses  the  gasometric  apparatus  shown 
on  p.  303,  the  amount  of  nitrogen  in  the  water  existing 
as  nitrates  and  nitrites  may  be  readily  and  accurately 
estimated  by  determining  the  volume  of  nitric  oxide 
evolved  on  agitating  the  concentrated  water,  acidulated 
with  strong  sulphuric  acid,  with  metallic  mercury.  This 
process,  which  is  an  adaptation  by  Frankland  of  Crum's 
method  for  the  refraction  of  nitre,  is  conducted  as  fol- 
lows :  The  residue  from  500  c.c.  of  the  water  is  dissolved 
in  a  small  quantity  of  hot  water,  the  chlorine  precipitated 
by  addition  of  a  slight  excess  of  silver  sulphate,  the 
liquid  filtered  and  concentrated  to  a  bulk  not  exceed- 
ing 2  c.c.,  and  is  then  transferred  to  the  apparatus  seen 
in  fig.  7  2 A.  This  consists  of  a  stout  tube  about  20  c.m. 


318  Quantitative  Chemical  Analysis. 

long  and  about  1*5  c.m.  internal  diameter,  fitted  with  a  stop- 
cock. The  tube  is  filled  with 
mercury  to  the  stopcock,  and  in- 
verted in  a  basin,  also  containing 
mercury.  The  concentrated  fil- 
trate is  then  brought  into  the 
little  cup  together  with  the  wash- 
ings, and  is  allowed  to  enter 
the  tube  by  cautiously  turning  the 
stopcock.  About  i^  times  the 
volume  of  the  aqueous  solution 
is  then  poured  into  the  cup,  and 
thence  allowed  to  pass  into  the 
tube,  which  is  then  firmly  closed 
by  the  moistened  thumb  held 

obliquely,  and  vigorously  shaken  for  about  five  minutes. 
On  the  completion  of  the  reaction  the  gas,  nitric  oxide,  is 
transferred  to  the  measuring  apparatus,  and  its  volume  de- 
termined. If  500  c.c.  of  water  have  been  used,  the  volume 
of  the  nitric  oxide  denotes  the  volume  of  nitrogen,  as  nitrates 
and  nitrites,  in  1000  c.c.,  since  nitric  oxide  contains  half  its 
volume  of  nitrogen.  If  considerable  quantities  of  nitrites 
are  present,  they  must  be  oxidised  to  nitrates  by  adding  a 
very  dilute  solution  of  potassium  permanganate  to  the  acidu- 
lated water  until  the  pink  colour  is  permanent.  Sodium 
carbonate  is  then  added  to  alkaline  reaction,  and  the  water 
is  evaporated  and  treated  in  the  manner  above  described. 

Estimation  of  Nitrites. — A  solution  of  meta-phenylene- 
diamine  (C6H8N2)  in  dilute  sulphuric  acid  gives  a  dark  red 
or  reddish  violet  colouration  with  small  quantities  of  nitrous 
acid,  which  may  be  made  the  basis  of  a  method  for  the  de- 
termination of  the  nitrites  present  in  natural  waters.  The 
method  requires — 

(i)  A  Solution  of  Metaphenylenediamine. — Dissolve  5 
grams  of  the  base  in  i  litre  of  water  containing 
a  slight  excess  of  sulphuric  acid. 


Water  A  nalysis.  319 

(2)  Dilute  Sulphuric  Add\  i  part  of  acid  to  2  of  distilled 

water. 

(3)  Standard  Solution  of  pure  Sodium  Nitrite. — Dissolve 

0*406  gram  of  silver  nitrite  in  boiling  distilled 
water,  and  add  a  hot  solution  of  pure  common  salt 
so  long  as  silver  chloride  is  precipitated.  Dilute 
to  i  litre,  allow  the  precipitate  to  settle,  and  make 
up  each  100  c.c.  of  the  clear  solution  to  i  litre, 
i  c.c.  of  this  solution  is  equivalent  to  o-oi  mgrm. 
of  N2O3.  The  solutions  must  be  kept  in  well- 
stoppered  bottles,  which  should  be  quite  full. 

(4)  Four  narrow   cylinders   of   colourless  glass  :    these 

should  be  of  such  size  that  100  c.c.  of  water  rise 
to  a  height  of  about  18  c. :  the  level  of  the  water 
should  be  marked  or  etched  on  the  glass.  Also 
several  graduated  pipettes  or  burettes. 

•Fill  one  of  the  cylinders  to  the  mark  with  the  water  to  be 
tested,  and  mix  with  i  c.c.  of  the  dilute  sulphuric  acid  and 
i  c.c.  of  the  solution  of  meta-phenylenediamine,  and  stir 
with  the  bulb  tube.  If  a  red  colour  appears  immediately 
the  amount  of  the  nitrite  is  probably  too  great  for  accurate 
comparison :  in  that  case  take  50  c.c.,  or  25,  or  even  10  c.c. 
of  the  water,  make  up  to  100  c.c.,  and  repeat  the  trial.  The 
colour  should  appear  only  at  the  expiration  of  70  or  80 
seconds. 

Into  the  other  three  cylinders  place  measured  quantities 
from  0*3  to  2-5  c.c.  of  the  standard  nitrite  solution,  dilute  to 
the  mark  with  distilled  water,  and  add  to  each  i  c.c.  of  the 
sulphuric  acid  and  meta-phenylene  diamine  solutions,  and 
compare  the  tints  formed  with  that  in  the  first  cylinder. 
This  comparison  give  a  first  approximation.  The  trials  are 
repeated  on  a  fresh  sample  of  the  water  and  with  the  quan- 
tities of  standard  nitrite  now  deemed  necessary  to  produce 
a  similar  tint.  Lastly,  a  final  series  of  comparisons,  starting 
simultaneously,  should  be  made,  and  the  colours  compared 
after  the  lapse  of  20  minutes. 


320  Quantitative  Chemical  Analysis. 

Estimation  of  Chlorine.  By  Standard  Silver  Nitrate  and 
Potassium  Chr ornate  Solutions. — Dissolve  2-3944  grm.  of 
pure  dry  silver  nitrate  in  distilled  water,  and  dilute  to  i  litre ; 
i  c.c.  of  this  solution  is  equivalent  to  0-5  milligram  of  chlo- 
rine. The  potassium  chromate  solution  should  be  strong 
and  neutral.  Transfer  50  c.c.  of  the  water  to  be  tested 
to  a  porcelain  basin,  colour  with  2  drops  of  the  potassium 
chromate  solution,  and  add  the  silver  solution,  drop  by 
drop,  until  the  permanent  red  colour  of  the  silver  chromate 
makes  its  appearance,  If  50  c.c.  have  been  taken,  the 
number  of  c.c.  of  the  silver  solution  employed  gives  the 
amount  of  chlorine  in  parts  per  100,000. 

In  many  highly-coloured  waters  the  final  point  of  the 
reaction  is  not  distinctly  visible.  In  such  a  case  add  a  small 
quantity  of  lime-water  (free  from  chlorides)  to  the  measured 
portion  of  the  water,  pass  washed  carbonic  acid  through  the 
liquid,  boil,  and  filter.  The  colouring  matter  will  in  general 
be  carried  down  by  the  precipitated  •  chalk  :  the  amount  of 
chlorine  in  the  filtrate  may  then  be  determined  as  directed 
above.  It  will  be  sometimes  necessary,  however,  to  destroy 
the  organic  matter  by  heat  :  the  measured  portion  of  the 
water,  after  addition  of  a  small  quantity  of  lime-water,  must 
be  evaporated  to  dryness  and  the  residue  gently  heated. 
Treat  the  saline  matter  with  hot  water,  filter,  and  determine 
the  chlorine  in  the  usual  manner. 

The  quantity  of  chlorine  contained  in  the  water  will  often 
afford  another  indication  of  its  purity.  Very  pure  waters,  as 
a  rule,  contain  comparatively  small  quantities  of  chlorine, 
less  than  i  part  in  100,000  ;  when  contaminated  with 
sewage  (which  contains  on  the  average  about  10  parts  in 
100,000),  the  quantity  is  largely  increased.  Of  course  in 
judging  of  the  character  of  a  drinking-water  from  the  amount 
of  chlorine  it  contains,  due  regard  must  be  had  to  the  nature 
of  the  strata  through  which  it  percolates.  Water  originating 
from  springs  in  the  neighbourhood  of  the  sea,  especially  if 
the  district  be  sandy,  may  contain  considerable  amounts  of 
chlorine,  and  yet  be  free  from  sewage-matter. 


Water  Analysis.    '  321 

Estimation  of '  Hardness! — Waters  are  familiarly  spoken 
of  as  *  hard  '  and  *  soft ' :  these  terms  have  reference  to  the 
action  of  soap  upon  the  water.  A  '  hard  '  water  necessitates 
the  expenditure  of  much  soap  before  it  will  give  a  lather  ; 
this  expenditure  is  caused  by  the  action  of  the  lime  and  mag- 
nesia salts,  which  decompose  the  soap,  or  stearate  of  soda, 
forming  insoluble  stearates  of  lime  and  magnesia.  These 
substances  constitute  the  pellicle  or  scum  which  forms  upon 
the  surface  of  a  hard  water  after  treatment  with  soap.  Before 
a  '  hard  '  water  can  give  a  lather  useful  for  detergent  action, 
sufficient  soap  must  be  used  to  convert  all  the  lime  and 
magnesia  salts  present  into  stearates.  The  estimation  of 
the  hardness  or  soap-destroying  power  of  the  water  becomes 
therefore  an  important  element  in  determining  its  value  for 
economic  purposes. 

The  soap-destroying  power  of  the  water  is  measured 
directly  :  a  solution  of  soap  of  known  strength  being  made  to 
act  upon  a  definite  volume  of  the  water  until  a  permanent 
(and  detergent)  lather  is  obtained. 

I.  Preparation  of  the  Strong  Soap  Solution. — Pound,   in 
small  quantities  at  a  time,  in  a  mortar,  3  parts  of  lead  plaster 
and  i  part  of  dry  potassium  carbonate.     Mix  thoroughly,  and 
add  a  small  quantity  of  methylated  spirit,  and  triturate  until  a 
thin  creamy  mixture  is  obtained.     After  standing  for  some 
hours,  pour  the  clear  solution  through  a  filter,  and  exhaust 
the  residue  repeatedly  with  fresh  portions  of  spirit.     If  the 
solution  remains  clear  on  standing,  proceed  to  determine  its 
exact  strength  by  the  aid  of  a  standard  solution  of  calcium 
chloride. 

II.  Preparation  of  Standard  Calcium  Chloride  Solution. — 
Weigh  out  into  a  porcelain  or  platinum  dish  exactly  *2  gram 
of  finely-powdered  marble,  cover  the  dish  with  a  large  watch- 
glass,  and  dissolve  the  marble  in  dilute  hydrochloric  acid. 
Heat  the  basin  on  the  water-bath,  and  when  the  expulsion  of 

Y 


322  Quantitative  Chemical  Analysis. 

the  carbon  dioxide  is  at  an  end,  rinse  the  under- surface  of 
the  watch-glass  into  the  basin  and  evaporate  to  complete 
dryness.  Add  a  small  quantity  of  water,  and  again  evaporate 
to  ensure  the  complete  removal  of  the  excess  of  the  hydro- 
chloric acid.  Dissolve  in  water,  and  dilute  to  1,000  c.c. 

III.  Dilution  of  the  Strong  Soap  Solution. — Transfer  50  c.c. 
of  the  standard  calcium  chloride  solution  to  a  bottle  of  25oc.c. 
capacity,  provided  with  a  well-fitting  stopper,  fill  up  a  burette 
with  the  soap  solution  and  add  it  to  the  water  in  the  bottle 
in  quantities  of  a  few  drops  at  a  time.  After  each  addition 
of  the  soap  solution  insert  the  stopper,  and  shake  briskly. 
The  process  is  finished  when  a  uniform  lather  is  obtained 
which  is  permanent  for  at  least  3  minutes,  and  which  may  be 
re-formed  by  again  shaking  the  liquid.  Read  off  the  burette 
and  dilute  the  soap  solution  with  a  mixture  of  2  vols.  of 
methylated  spirit  and  i  vol.  of  water,  until  about  12  c.c.  of 
the  diluted  mixture  are  equivalent  to  50  c.c.  of  the  standard 
calcium  chloride  solution;  allow  it  to  stand  for  24  hours, 
filter  it  if  necessary,  again  determine  its  strength,  and  dilute 
it  with  the  mixture  of  alcohol  and  water,  until  exactly  14*25 
c.c.  are  required  to  produce  a  permanent  lather  with  50  c.c. 
of  the  standard  calcium  chloride  solution. 

The  Process. — Transfer  50  c.c.  of  the  water  under  examina- 
tion to  the  250  c.c.  bottle,  shake  vigorously,  and  suck  out 
the  air  from  within  the  bottle  by  the  aid  of  a  glass  tube,  to 
remove  the  carbon  dioxide  expelled  on  agitating.  Fill  up 
the  burette  with  the  soap  solution,  and  add  it,  i  c.c.  at  a 
time,  to  the  water;  after  each  addition  insert  the  stopper,  and 
shake  vigorously.  As  soon  as  a  froth  begins  to  form,  add 
the  solution  of  soap  in  smaller  quantities  until  a  uniform  per- 
manent lather  is  obtained.  If  more  than  16  c.c.  of  the 
soap  solution  are  required  the  operation  must  be  repeated  on 
a  smaller  quantity  of  the  water.  Transfer  25  c.c.,  or  less  if 
it  is  very  hard,  of  the  water  to  the  bottle,  and  add  sufficient 
distilled  water  to  make  up  the  volume  to  50  c.c.,  and  again 


Water  A  nalysis. 


323 


add  the  soap  solution,  in  small  quantities  at  a  time,  until  the 
lather  is  obtained.  Multiply  the  volume  in  c.c.  of  soap 
solution  used  by  the  number  expressing  the  fraction  of  50  c.c. 
taken  :  thus,  if  25  have  been  taken,  multiply  by  2  :  if  10, 
multiply  by  5.  The  weight  of  calcium  carbonate  in 
100,000  parts  of  water,  corresponding  to  the  number  ofc.c. 
required  for  50  c.c.  of  the  water,  is  given  in  the  following 
table  :  Column  I.  gives  volume  of  soap  solution  :  Column 
II.  the  corresponding  amount  of  calcium  carbonate  per 
100,000  parts. 

Table  of  Hardness. 


i 

II. 

L 

II. 

I. 

II. 

I. 

II. 

I 

II. 

I. 

II. 

c.c. 

07 

•oo 

c.c. 

3-3 

3-64 

c.c. 

5  "9 

7-29 

C.C. 

8'5 

11-05 

C.C. 

II'I 

15-00 

c.c. 

137 

19-13 

•8 

•16 

•4 

77 

6-0 

•43 

•6 

•20 

•2 

•16 

•8 

•29 

•9 

•32 

•5 

•90 

*i 

'57 

7 

'35 

'3 

•32 

•9 

•44 

I'O 

•48 

•6 

4-03 

•2 

•71 

•8 

•50 

•4 

•48 

14-0 

•60 

•I 

•63 

7 

•16 

'3 

•86 

'9 

•65 

•5 

•63 

•i 

76 

'2 

79 

•8 

•29 

'4 

8-00 

9-0 

•80 

•6 

79 

'2 

•92 

'3 

'95 

'9 

•43 

'5 

'14 

•95 

7 

'95 

'3 

20-08 

'4 

i-ii 

4'0 

'57 

•6 

•29 

•2 

12-11 

•8 

16-11 

'4 

•24 

I 

•27 
•43 

•i 

'2 

71 

•86 

•8 

'43 

'57 

"3 
'4 

•26 
•41 

'9 

I2'O 

•27 
'43 

| 

•40 

•56 

7 

•56 

'3 

5  -oo 

'9 

•71 

•5 

•56 

'I 

'59 

7 

•71 

•8 

•69 

'4 

•14 

7-0 

•86 

•6 

•71 

•2 

75 

•8 

•87 

•9 

•82 

•5 

•29 

•i 

9-00 

7 

•86 

•3 

•90 

'9 

21-03 

2'0 

'95 

•6 

•43 

•2 

•14 

•8 

13-01 

'4 

17-06 

15-0 

•19 

•I 

2-08 

7 

•57 

'3 

•29 

'9 

•16 

•15 

•22 

'35 

'2 

•21 

•8 

7i 

'4 

'43 

10  -o 

•3i 

•6 

•38 

'2 

'Si 

'3 

'34 

•9 

•86 

•5 

'57 

•I 

•46 

7 

'54 

'3 

•68 

"4 

'47 

5'° 

6-00 

•6 

•71 

•2 

•6  1 

•8 

70 

'4 

•85 

'5 

•60 

•i 

•14 

7 

•86 

'3 

•76 

'9 

•86 

'5 

22-02 

•6 

73 

•2 

•29 

•8 

lO'OO 

•4 

•91 

13-0 

18-02 

•6 

•18 

7 

•86 

'3 

'43 

'9 

•15 

•5 

14-06 

•i 

•17 

7 

•35 

•8 

•99 

•4 

•57 

8-0 

•30 

•6 

•21 

•2 

•33 

•8 

•52 

'9 

3-12 

'5 

71 

•i 

•45 

7 

'37 

'3 

*49 

'9 

•69 

3-0 

•25 

•6 

•86 

•2 

•60 

•8 

•52 

'4 

•65 

16-0 

•86 

•i 

•38 

7 

7-00 

'3 

75 

•9 

•68 

•5 

•81 

•2 

'Si 

•8 

•14 

'4       -90 

II'O 

•84 

•6 

•97 

In  waters   rich  in   magnesian-salts  the  lather  acquires  a 
characteristic  curdy  appearance,  easily  recognised  after  a 


Y2 


324  Quantitative  Chemical  Analysis. 

little  experience.  To  familiarise  himself  "with  the  difference 
in  the  lathers  occasioned  by  calcareous  and  magnesian  waters, 
the  student  should  make  a  dilute  solution  of  magnesium  sul- 
phate in  strength  equal  to  that  of  the  standard  calcium  chloride 
solution,  and  compare  the  lathers  obtained  by  adding  a  slight 
excess  of  soap  solution  to  equal  volumes  of  the  two  liquids. 
If  a  trial  has  shown  the  presence  of  magnesia  salts  in  large 
proportion,  the  experiment  should  be  repeated,  with  the 
water  so  far  diluted  with  distilled  water,  that  50  c.c.  of  the  mix- 
ture require  only  about  7  c.c.  of  the  standard  soap  solution. 

It  is  well  known  that  the  hardness  of  water  is  occasionally 
diminished  by  boiling :  such  a  water  contains  magnesium  and 
calcium  carbonates,  dissolved  in  free  carbonic  acid.  On  boil- 
ing the  water  the  free  carbonic  acid  is  expelled  and  the  car- 
bonates are  almost  entirely  precipitated,  no  diminution  of 
the  hardness  will  occur  on  boiling,  unless  it  exceeds  three  parts 
per  100,000,  since  the  carbonates  are  dissolved  to  that  extent 
by  water  free  from  carbonic  acid.  It  sometimes  happens 
that  the  hardness,  even  when  considerable,  is  not  lessened 
by  boiling :  in  this  case  it  is  due  to  calcium  and  magnesium 
sulphates  or  chlorides  :  such  a  water  is  termed  permanently 
hard  :  a  water  which  owes  its  soap- destroying  power  to  car- 
bonates is  said  to  be  temporarily  hard.  It  is  generally  desira- 
ble to  distinguish  between  temporary  and  permanent  hardness 
in  the  analysis.  For  this  purpose  transfer  200  c.c.  of  the  water 
to  a  flask,  and  heat  to  boiling.  After  half  an  hour's  gentle 
ebullition,  remove  the  lamp,  allow  the  water  to  cool  slightly, 
filter  it  through  a  small  filter,  make  its  volume  up  to  200  c.c., 
and  again  determine  the  hardness  on  50  c.c.  of  the  filtrate. 

The  number  of  c.c.  of  soap  solution  employed  shows  the 
permanent  hardness,  and  this  subtracted  from  that  of  the 
unboiled  water  gives  the  temporary  hardness.  Sometimes, 
although  rarely,  the  hardness  of  the  water  is  in  part  due  to 
free  hydrochloric  or  sulphuric  acids ;  it  is  advisable,  there- 
fore, to  ascertain  its  reaction  before  determining  its  soap- 
destroying  power. 


Water  Analysis.  325 

The  hardness  of  water  is  of  importance  in  determining  its 
value  for  manufacturing  purposes.  Hard  waters  are  a  source 
of  much  annoyance  to  the  manufacturer.  The  hard  crust  or 
1  cake '  which  forms  in  steam  boilers  consists  of  sulphate  of 
lime  and  carbonates  of  calcium  and  magnesium,  often  mixed 
with  co-precipitated  organic  matter. 

Detection  of  Lead  and  Copper. — Concentrate  a  litre  of  the 
water  to  about  50  c.c.,  and  filter  it,  if  necessary,  into  one 
of  the  cylinders  used  for  the  estimation  of  the  ammonia : 
add  two  or  three  drops  of  acetic  acid,  and  2  c.c.  of  a 
freshly-prepared  and  saturated  solution  of  sulphuretted  hy- 
drogen. If  a  brown  colouration  is  produced  fill  a  second 
cylinder  with  distilled  water,  acidulate  with  two  or  three 
drops  of  acetic  acid,  mix  with  2  c.c.  of  the  sulphuretted  hy- 
drogen solution,  and  add  a  standard  solution  of  lead  con- 
taining TV  milligram  per  c.c.  (obtained  by  dissolving  0-1831 
gram  of  crystallised  lead  acetate  in  a  litre  of  distilled  water) 
until  the  colouration  is  equal  in  both  tubes.  Copper  may  be 
accurately  estimated  by  means  of  a  solution  of  potassium 
ferrocyanide.  The  reddish-brown  tint  thus  produced  is  com- 
pared with  that  formed  under  similar  circumstances  in  dis- 
tilled water  containing  a  known  quantity  of  copper.  Iron 
may  be  determined  by  a  similar  process.  (Carnelley,  Chem. 
News,  XXXII.  308.) 

To  determine  if  the  water  has  any  action  on  lead  fill  two 
beakers  with  the  sample ;  into  one  place  a  bright  strip  of 
the  metal,  and  into  the  other  a  strip  which  has  been  tarnished 
by  previous  exposure  to  water,  and  leave  them  in  contact 
with  the  sample  for  24  hours.  The  strips  are  then  removed 
and  the  water  tested  in  the  manner  above  described.  Many 
waters  which  rapidly  attack  a  clean  surface  of  lead  have  no 
action  on  the  tarnished  metal :  other  waters,  especially  when 
containing  nitrates  and  nitrites,  act  on  lead  whether  bright 
or  tarnished. 

The  estimation  of  the  amounts  (i)  of  ammonia,  (2)  of 
organic  carbon  and  nitrogen  (or,  if  preferred,  of  the  *  albu- 


3  26  Quantitative  Chemical  A  nalysis. 

minoid  ammonia '),  (3)  of  nitrogen  as  nitrates  and  nitrites, 
(4)  of  chlorine,  (5)  of  total  soluble  and  suspended  matter, 
(6)  of  the  hardness,  and  (7)  of  the  presence  or  absence  of 
lead,  afford  the  principal  data  in  determining  the  value  of  a 
sample  of  water  for  domestic  supply. 

Occasionally  it  is  necessary  to  ascertain  more  particularly 
the  nature  of  the  inorganic  matter  in  solution :  the  brewer, 
for  example,  frequently  wishes  to  know  the  actual  amount  of* 
the  calcium  and  magnesium  sulphates  and  carbonates  present. 
The  estimation  of  the  various  inorganic  constituents  may  be 
readily  made  by  methods  already  described  at  length. 

Estimation  of  Silica. — Evaporate  not  less  than  a  litre  of 
the  water  to  dryness  (best  in  a  platinum  dish),  after  acidifying 
with  a  few  drops  of  hydrochloric  acid.  Dry  the  saline 
residue  thoroughly,  moisten  with  hydrochloric  acid,  dilute 
with  hot  water,  and  filter  off  the  separated  silica. 

Estimation  of  Iron. — Add  two  drops  of  nitric  acid  to  the 
filtrate  from  the  silica,  boil  and  add  ammonium  chloride  and 
a  slight  excess  of  ammonia,  allow  the  precipitate  to  settle, 
pour  the  supernatant  liquid  through  a  small  filter,  redissolve 
the  precipitate  in  the  least  possible  quantity  of  hydrochloric 
acid,  and  again  add  ammonia.  Transfer  the  precipitate  to 
the  filter,  wash  with  hot  water,  dry,  and  weigh  the  ferric  oxide. 

Estimation  of  Lime. — Add  excess  of  ammonium  oxalate  to 
the  filtrate  from  the  preceding  estimation,  filter  the  precipi- 
tated calcium  oxalate,  and,  after  washing  and  drying,  ignite 
it  strongly  and  weigh  as  caustic  lime. 

Estimation  of  Magnesia. — Concentrate  the  filtrate  from 
the  calcium  oxalate,  add  sodium  phosphate  and  ammonia, 
and  treat  the  magnesium-ammonium  phosphate  in  the  usual 
manner. 

Estimation  of  Sulphuric  Acid. — Acidify  a  litre  of  the  water 
with  a  few  drops  of  hydrochloric  acid,  concentrate  to  80  or 
loo  c.c.  and  add  excess  of  barium  chloride  solution.  Filter 
off  the  barium  sulphate  and  weigh  it 


Gases  in  Water.  327 

Estimation  of  Phosphoric  Acid. — Many  samples  of  water 
rich  in  lime  and  magnesia  salts  contain  estimable  quantities 
of  this  acid.  To  determine  its  amount  acidify  a  litre  of  the 
water  with  nitric  acid,  concentrate  to  50  c.c.,  and  add  solu- 
tion of  molybdic  acid  (p.  218).  After  standing  for  24  hours 
treat  the  yellow  precipitate  as  directed  on  p.  219. 

Estimation  of  Sodium  and  Potassium. — Add  a  few  drops 
of  barium  chloride  to  a  litre  of  the  water,  to  precipitate  the 
sulphuric  acid,  and  boil  with  pure  milk  of  lime  to  throw 
down  the  magnesia,  iron,  and  phosphoric  acid.  Filter,  con- 
centrate the  filtrate,  add  ammonia,  ammonium  carbonate, 
and  a  few  drops  of  ammonium  oxalate ;  again  filter,  and 
evaporate  the  filtrate  to  dryness,  ignite  to  expel  ammoniacal 
salts  :  treat  the  residue  with  a  small  quantity  of  water,  filter, 
if  necessary,  acidify  with  hydrochloric  acid,  and  evaporate  to 
dryness  in  a  weighed  platinum  dish.  The  alkalies  may  then 
be  separated  by  platinum  tetrachloride,  or  their  proportion 
may  be  determined  by  dilute  standard  silver  solution  and 
potassium  chromate. 

XLI.  DETERMINATION  OF  THE  AMOUNT  AND  NATURE  OF 

THE   GASES   DISSOLVED    IN   WATER. 

Pure  natural  water  (rain  water)  when  thoroughly  aerated 
contains  about  2073  c.c.  of  gases  per  litre,  composed  of 

Nitrogen         ....      13*08  c.c. 
Oxygen.         ....       6*37 
Carbon  dioxide       .         .         .       1*28 

2073 

The  ratio  of  the  oxygen  to  the  nitrogen  is  as  i  :  2-05. 

If  the  water  becomes  contaminated  with  putrescent 
organic  substances  the  quantity  of  oxygen  rapidly  diminishes. 
It  is  abstracted  from  the  water  in  oxidising  the  carbon, 
hydrogen,  and  nitrogen  of  the  organic  matter.  By  deter- 
mining the  ratio  of  the  oxygen  to  the  nitrogen  in  the  gases 
which  it  contains,  we  may  often  ascertain  whether  such 
putrefactive  changes  are  in  actual  operation  in  the  water. 


328 


Quantitative  Chemical  A  nalysis. 


Several  methods  have  been  proposed  for  expelling  the 
gases  contained  in  water  :  *  one  of  the  simplest  of  these  is 
due  to  Reichardtf  Fig.  73  represents  the  apparatus  re- 
quired for  this  method.  A  is  an  ordinary  flask  of  from  800  to 
1,000  c.  c.  Its  capacity  must  be  accurately  known.  The 
narrow  cylindrical  vessel  B  serves  as  a  gasholder.  It  is 
fitted  with  a  caoutchouc  stopper  pierced  with  three  holes, 
through  one  of  which  passes  the  bent  tube  a,  the  longer 
limb  of  which  ends  in  B  at  about  one-third  of  its  height 

FIG.  73. 


from  the  bottom  :  the  other  end  is  fitted  into  the  caoutchouc 
stopper  of  A.  The  second  hole  in  the  stopper  of  B  contains 
the  tube  b,  which  passes  nearly  to  the  bottom  of  the  bottle : 
b  is  connected  by  means  of  a  caoutchouc  tube,  which  can  be 
closed  by  a  clamp,  with  the  glass  tube  c,  which  runs  nearly 
to  the  bottom  of  c.  The  third  hole  of  the  stopper  of  B 
contains  the  tube  d,  the  end  of  which  must  be  on  a  level 
with  the  under  surface  of  the  cork  :  d  is  connected  with  the 

*  See  Bunsen's  Gasometry,  translated  by  Roscoe  ;  Miller's  Inorganic 
Chemistry,  p.  187;  M'Leod,  Chem.  Soc.  Jour. 

f  Fresenius,  Zeits.  fur  Anal.  Chemie,  vol.  xi.  p.  271. 


Gases  in  Water.  329 

narrow  delivery  tube  ^,  terminating  beneath  the  surface  of 
the  mercury  in  the  trough,  in  which  is  supported  the  ab- 
sorption tube  T,  destined  to  receive  the  gases  from  the 
water,  and  which  is  therefore  filled  with  mercury  before  the 
commencement  of  the  experiment. 

The  flask  A  is  completely  filled  with  the  water  to  be 
examined  ;  B  and  c  are  also  partially  filled  with  recently 
well-boiled  and  still  warm  distilled  water,  and  b  and  ,t,  d  and 
e,  are  connected  together ;  both  the  clamps  being  open.  By 
simply  blowing  through/  the  bottle  B  and  all  the  tubes  are 
completely  filled  with  the  warm  water  :  the  clamps  of  b  and 
d  are  now  successively  closed,  and  the  tube  a  is  inserted 
into  the  flask  A,  care  of  course  being  taken  that  no  air- 
bubbles  lodge  beneath  the  surface  of  the  cork.  Open  the 
clamp  of  b  and  c,  and  gradually  boil  the  water  in  A.  The 
gas  expelled  on  ebullition  collects  in  B  :  as  soon  as  the 
whole  is  eliminated,  which  will  require  at  least  an  hour,  and 
the  water  in  B  is  nearly  boiling,  open  the  clamp  of  d  and  <?, 
cautiously  blow  through  f,  so  as  to  expel  the  water  con- 
tained in  d  and  <?,  which  is  allowed  to  flow  out  into  the  trough, 
and  displace  the  gases  from  B  into  the  measuring  tube  by 
again  blowing  through  f.  On  removing  the  lamp  from  beneath 
A,  the  water,  on  receding,  should  completely  fill  the  flask.  If 
it  does  not,  continue  the  ebullition  and  collect  the  small 
portion  of  the  remaining  gas. 

Allow  the  moist  gas  in  the  tube  T  to  acquire  the  temper- 
ature of  the  air  (obtained  from  the  thermometer  /).  Read  off 
the  level  of  the  mercury  within  and  without  the  tube, 
together  with  the  temperature  and  barometric  pressure  at 
the  time  of  observation,  and  calculate  the  volume  of  the 
gas  when  dry  at  o°  and  i  metre  pressure.  Introduce  a  ball 
of  potash  attached  to  a  platinum  wire  into  the  tube,  and  in 
six  or  eight  hours  again  read  off  the  levels  of  the  mercury. 
Quickly  withdraw  the  piece  of  potash,  moisten  it  slightly 
with  water,  and  reintroduce  it  into  the  tube,  and  at  the 
expiration  of  another  hour  again  determine  the  levels  of  the 


3  3°  Quart  titative  Chemical  A  na lysis. 

mercury,  the  temperature,  and  atmospheric  pressure.  If 
the  second  and  third  readings  are  identical  the  absorption 
of  the  carbon  dioxide  is  complete.  Reduce  the  volume  to 
o°  and  i  metre  pressure  :  it  is  not  necessary  to  deduct  the 
tension  of  aqueous  vapour  corresponding  to  the  tempera- 
ture observed,  since  the  gas  may  be  assumed  to  be  dry 
after  having  been  in  contact  with  the  potash.  Bring  into 
the  tube  a  papier-mache  bullet,  soaked  with  a  solution  of 
potassium  pyrogallate,  and  from  time  to  time  read  off  the 
level  of  the  mercury  within  the  tube.  As  soon  as  the  con- 
traction appears  to  be  finished  withdraw  the  papier-mache' 
bullet,  and  accurately  determine  the  position  of  the  levels 
of  the  mercury  within  and  without  the  tube,  together  with 
the  temperature  and  pressure.  Reduce  the  volume  of  the 
residual  gas  (nitrogen)  to  o°C  and  i  metre  pressure.  The 
volume  absorbed  by  the  alkaline  pyrogallate  represents 
the  amount  of  oxygen. 

The  potash  balls  for  the  absorption  of  carbon  dioxide  may 
be  readily  made  by  inserting  the  end  of  a  platinum  wire 
into  a  little  notch  filed  in  the  face  of  a  bullet-mould,  on  the 
opposite  side  to  the  orifice  through  which  in  bullet-making 
the  fused  lead  is  poured.  A  small  quantity  of  potash  is 
melted  in  a  silver  dish  and  poured  through  the  hole  until 
the  mould  is  completely  filled.  When  quite  cold  the  potash 
ball  may  be  readily  detached  from  the  metal.  The  short 
projecting  piece  formed  by  the  hole  through  which  the 
potash  has  been  poured  should  be  cut  away  and  the  ball 
preserved  in  a  stoppered  bottle  until  used. 

The  papier-mache'  balls  are  made  in  a  similar  way :  filter- 
paper,  converted  into  pulp  by  maceration  in  water,  is  forced 
through  the  hole  into  the  mould  until  it  is  quite  filled,  and 
the  mould  is  placed  in  the  steam  chamber  and  heated  until 
the  paper  is  dry. 

Of  course  if  the  operator  possesses  the  apparatus  described 
on  p.  303  et  sey.y  the  accurate  analysis  of  the  gas  by  the  use 
of  liquid  reagents  becomes  the  work  of  a  few  minutes  only. 


Manures.  331 

MANURES. 

XLII.      GUANO. 

THIS  substance  is  the  excrement  of  sea-birds,  more  or  less 
altered  by  exposure  to  the  weather.  It  contains  ammonia 
in  combination  with  uric,  oxalic,  carbonic,  and  phosphoric 
acids,  phosphates  and  sulphates  of  lime,  magnesia,  and 
alkalies,  and  more  or  less  organic  matter,  water,  sand,  &c. 
It  is  very  variable  in  composition,  and  is  often  largely 
adulterated.  Its  fertilising  power  mainly  depends  upon  the 
ammonia  and  phosphoric  acid  which  it  contains. 

Mix  the  sample  carefully,  and  transfer  about  50  grams  to 
a  stoppered  bottle  ;  the  several  portions  used  in  the  analysis 
are  to  be  taken  from  this  quantity. 

i.  Determination  of  the  Moisture. 

Weigh  out  about  5  or  6  grams  of  the  guano  into  the  tube, 
fig.  29,  p.  70,  heat  the  oil-bath  to  120°,  aspirate  a  slow 
current  of  dry  air  through  the  apparatus,  and  repeatedly 
weigh  the  tube  until  the  weight  is  constant.  The  flask 
contains  5  c.c.  of  normal  acid,  diluted  with  water,  to  absorb 
the  ammonia,  which  volatilises  with  the  steam  when  guano  is 
heated  ;  the  quantity  of  residual  acid  may  be  determined 
with  litmus  and  a  dilute  soda  solution  in  the  usual  way. 
The  loss  in  the  weight  of  the  tube,  minus  the  amount  of 
ammonia  retained  in  the  flask,  gives  the  quantity  of  moisture. 

2.  Determination  of  Fixed  Inorganic  Matter. 

About  four-fifths  of  the  dried  guano  is  transferred  from  the 
tube  to  a  weighed  platinum  dish.  Re-cork  the  tube  securely  ; 
the  remaining  portion  of  the  dried  guano  will  be  used  for  the 
estimation  of  the  nitrogen.  Gently  ignite  the  portion  in  the 
dish.  The  ash  should  be  nearly  white ;  if  it  is  of  a  reddish 
colour,  adulteration  with  sand  or  clay  may  be  suspected. 
The  quantity  of  the  ash  in  the  better  class  of  guanos  does 


332  Quantitative  Chemical  Analysis. 

not  exceed  35  per  cent.   The  loss  of  weight  gives  the  organic 
matter,  together  with  the  ammonia  and  combined  water. 

3.  Determination  of  the  Insoluble  Matter,  Sand,  &°<r. 

Boil  the  weighed  portion  of  the  ash  with  dilute  nitric  acid 
for  15  minutes.*  If  the  substance  effervesces  strongly  the 
guano  has  in  all  probability  been  adulterated  with  calcium 
carbonate.  Heat  on  the  water-bath  for  some  time,  add  water, 
filter  into  a  300  c.c.  flask,  wash  the  residue,  dry,  and  weigh  it. 

4.  Determination  of  the  Phosphoric  Acid,  Lime,  Magnesia, 
Sulphuric  Acid,  and  Alkalies. 

Make  up  the  filtrate  to  the  containing-mark,  and  shake 
the  liqui'd. 

(a)  Phosphoric  Acid,  Lime,  and  Magnesia. — Transfer 
100  c.c.  of  the  solution  to  a  -J-litre  flask,  add  excess  of 
ammonia,  and  then  acetic  acid  to  acid  reaction.  Dilute  the 
liquid  to  the  mark,  and  shake. 

a.  Phosphoric  Acid. — By  Standard  Uranium  Solution. — 
When  a  solution  of  acetate  or  nitrate  of  uranium  is  added 
to  a  solution  of  phosphoric  acid,  containing  ammoniacal 
salts  and  free  acetic  acid,  a  light  greenish-yellow  precipitate 
of  the  double  uranium-ammonium  phosphate  is  produced. 
This  precipitate  is  insoluble  in  water,  and  in  acetic  acid ; 
the  stronger  acids,  however,  dissolve  it. 

A  solution  of  the  acetate  or  nitrate  of  uranium  gives  a 
reddish-brown  colour  with  potassium  ferrocyanide.  This 
colour  is  not  produced  so  long  as  any  phosphoric  acid  is  in 
solution.  These  reactions  form  the  basis  of  an  accurate 
volumetric  method  for  the  estimation  of  phosphoric  acid. 

Preparation  of  the  Standard  Solution  of  Uranium. — 
Dissolve  about  35  grams  of  well-crystallised  uranium 
nitrate  or  acetate  (the  former,  however,  is  preferable)  in 
900  c.c.  of  water.  The  solution,  mixed  with  sodium 
acetate,  must  be  standardised  by  a  solution  of  pure  sodium 

*  In  burning  off  the  organic  matter  the  phosphates  are  partially 
converted  into  pyrophosphates.  By  boiling  with  dilute  nitric  acid  the 
pyvophosphates  are  reconverted  into  orthophosphates. 


Manures.  333 

phosphate  of  known  strength.  Dissolve  100  grams  of 
sodium  acetate  in  900  c.c.  of  water,  and  dilute  with  strong 
acetic  acid  to  i  litre.  Dissolve  10-085  grams  of  pure  and 
non-effloresced  crystals  of  sodium  phosphate,  previously 
dried  by  pressure  between  filter-paper,  in  a  litre  of  water. 

Transfer  50  c.c.  of  the  sodium  phosphate  solution,  corre- 
sponding to  o-i  gram  P2O5,  to  a  beaker,  add  5  c.c.  of  the 
sodium  acetate  solution,  and  heat  the  mixture  on  the  water- 
bath.  Remove  it  when  its  temperature  is  about  80°,  and 
quickly  run  in  10  or  12  c.c.  of  the  uranium  solution,  with 
constant  stirring.  Now  add  the  uranium  solution  more 
cautiously,  in  quantities  of  0^5  c.c.  at  a  time,  and  test  the 
mixture  after  each  addition.  For  this  purpose  bring  a  few 
drops  of  the  turbid,  but  nearly  colourless,  liquid,  on  to  a 
porcelain  slab,  and  add  to  it  a  small  drop  of  potassium  ferro- 
cyanide  solution;  if  the  least  excess  of  uranium  salt  be 
present,  the  mixed  drops  will  acquire  a  reddish-brown 
colour.  If  this  colour  is  not  at  once  perceived,  continue  the 
addition  of  the  uranium  solution  until  it  just  appears. 
Replace  the  beaker  on  the  water-bath,  and  in  a  few  minutes 
again  transfer  a  few  drops  to  the  slab,  and  test  a  second  time 
with  the  ferrocyanide.  If  the  colouration  is  still  distinctly  visi- 
ble, the  process  is  finished.  The  solution  of  the  uranium  salt 
must  now  be  diluted,  until  20  c.c.  are  exactly  required  for  the 
50  c.c.  of  sodium  phosphate  solution  :  20  c.c.  thus  become 
equivalent  to  0*1  gram  of  P2O5,  or  i  c.c.  =  5  milligrams 
P2O5.  The  exact  strength  should  be  again  determined  by 
several  trials  after  the  dilution,  has  been  effected. 

To  apply  this  process  to  the  determination  of  the  phos- 
phoric acid  contained  in  the  solution  of  the  guano,  transfer 
50  c.c.  of  the  acetic  acid  solution  (a)  of  the  phosphate  to  a 
beaker,  heat  on  the  water-bath,  and  proceed  exactly  as 
described.  Repeat  the  determination  on  a  second  portion 
of  the  liquid. 

/3.  Lime. — Transfer  100  c.c.  of  the  liquid  (a)  to  a  beaker, 
heat,  and  add  excess  of  ammonium/  oxalate ;  the  calcium 


334  Quantitative  Chemical  Analysis. 

oxalate  is  filtered  off  after  standing,  and  weighed  as  carbo- 
nate, or  it  is  treated  with  sulphuric  acid,  and  titrated  with 
potassium  permanganate. 

y.  Magnesia.  The  filtrate  from  the  lime  precipitate  is 
mixed  with  ammonia  and  sodium  phosphate,  and  the  mag- 
nesium-ammonium phosphate  weighed  as  pyrophosphate. 

(b}  Determination  of  Sulphuric  Acid  and  Alkalies. — 
Transfer  the  remainder  (200  c.c.)  of  the  liquid  in  4  to  a 
beaker,  heat,  add  barium  chloride,  and  filter  off  any  barium 
sulphate  which  may  form.  To  the  filtrate  add  milk  of  lime 
or  baryta  water,  boil,  filter,  and  precipitate  the  excess  of  the 
alkaline  earth  with  ammonia  and  ammonium  carbonate,  filter, 
evaporate  to  dryness,  and  ignite,  to  expel  the  ammoniacal 
salts.  The  residue  is  treated  with  a  small  quantity  of  water, 
filtered  again,  if  necessary,  and  the  filtrate  evaporated  to 
dryness  in  a  weighed  platinum  dish.  The  proportion  of  the 
alkalies  in  the  washed  chlorides  may  then  be  determined  by 
a  dilute  standard  solution  of  silver  nitrate  and  potassium 
chromate. 

5.  Determination  of  the  Nitrogen. 

a.  Nitrogen  existing  as  Ammonia. — From  i  to  5  grams 
of  the  guano,  according  to  its  supposed  richness  in  ammo- 
nia, as  determined  by  the  amount  evolved  in  drying,  are 
weighed  out  into  the  retort,  fig.  32,  and  boiled  with  magnesia 
for  some  time.  The  ammonia  is  gradually  expelled,  and 
may  be  collected  in  the  flask  c,  containing  a  known  quan- 
tity of  standard  sulphuric  or  hydrochloric  acid,  diluted  with 
water.  The  quantity  of  the  residual  free  acid  is  then  to  be 
estimated  by  standard  soda  and  litmus  solution.  The  use 
of  caustic  soda  or  lime  in  expelling  the  ammonia  is  in- 
admissible, since  these  substances  would  convert  a  portion 
of  the  nitrogenous  organic  matter  into  ammonia. 

ft.  Nitrogen  existing  as  Azotised  Organic  Matter,  Uric 
Acid,  &>c.— By  Ignition  with  Soda-lime. — Many  organic  sub- 
stances containing  nitrogen,  not  in  the  form  of  nitroxides. 


Manures.  335 

when  heated  with  a  caustic  alkali,  give  up  the  whole  of 
their  nitrogen  in  the  form  of  ammonia.     This  reaction  con- 
stitutes the  principle  of  a  convenient  method  for  estimating 
the  amount  of  organic  nitrogen  existing  in  manures. 
The  following  articles  are  required  for  this  method  : — 

(a)  Combustion-tube. — This  should  have  the  form  seen  in 
fig.  74;  it  is  about  40  centimetres  long,  and  from  10  to  12 
millimetres  in  internal  diameter. 

(b)  Soda-lime. — Heat  a  sufficient  quantity  of  the  coarsely 
powdered  substance  in  a  porcelain  basin,  just  before  it  is 
wanted,  and  allow  it  to  cool.     A  mixture  of  equal  weights  of 
dry  slaked  lime  and  dehydrated  sodium  carbonate  may  be 
used  instead  of  soda- lime. 

(c)  Oxalic  Acid,— This  should  be  well  dried  in  the  water- 
bath  so  as  to  expel  all  its  water  of  crystallisation. 

(d)  Asbestos. — Ignite  a  small  quantity  in  the  gas-flame 
before  use. 

(e)  A  bulbed  U-tube,  fitted  with  caoutchouc  stopper  and 
bent  tube.    On  the  end  of  the  bent  tube  is  a  cork,  which 
fits  tightly  into  the  combustion-tube. 

1  Introduce  a  layer,  about  3  centimetres  in  length,  of  the 
dried  oxalic  acid,  mixed  with  a  small  quantity  of  soda-lime, 
into  the  posterior  end  of  the  tube,  and  afterwards  an 
equal  bulk  of  soda-lime.  Weigh  out  from  the  tube  the 
remainder  of  the  dried  guano  obtained  in  i,  into  a  dry 
porcelain  mortar,  and  mix  it  with  soda-lime.  Bring  the 
mixture  without  loss  of  time  (since  it  is  apt  to  part  with  a 
small  quantity  of  ammonia)  into  the  tube,  and  rinse  out  the 
mortar  with  a  fresh  portion  of  soda-lime.  The  substance 
should  be  mixed  with  a  sufficient  amount  of  soda-lime  to 
occupy  about  20  centimetres  of  the  length  of  the  tube. 
Fill  up  the  tube  to  within  5  centimetres  of  its  length  with 
soda-lime,  and  insert  a  loosely-fitting  plug  of  the  re- 
cently-ignited asbestos.  Fit  in  the  cork  of  the  U-tube, 
transfer  10  c.c.  of  standard  acid  to  the  U-tube,  and  add 
sufficient  water  to  fill  the  bulbs  to  the  extent  indicated  in 
the  figure.  Gently  tap  the  combustion-tube  on  the  table 


3  3^  Quantitative  Chemical  A  nalysis. 

so  as  to  make  a  passage  for  the  evolved  gases.  Place 
the  combustion- tube  in  the  furnace,  and  gradually  heat  it 
along  its  entire  length,  beginning  at  the  end  nearest  the 
U-tube.  The  heat  must  be  sufficient  to  cause  a  steady 
evolution  of  gas ;  towards  the  end  it  should  be  increased,  to 
break  up  any  cyanides  which  may  have  been  formed.  Do 
not  heat  the  extreme  end  of  the  tube  where  the  oxalic  acid 
is  situated,  until  the  evolution  of  the  gas  has  almost 
finished.  When  the  combustion  is  at  end,  and  the  evolution 
of  gas  has  totally  ceased,  cautiously  heat  the  oxalic  acid  ; 
this  occasions  a  brisk  current  of  carbon  dioxide,  which 
sweeps  out  all  the  ammonia  remaining  in  the  tube.  Remove 
the  U-tube  when  the  evolution  of  the  gas  has  nearly  finished, 

FIG.  74. 


add  a  few  drops  of  litmus  solution,  and  dilute  caustic 
soda  solution  from  a  burette,  until  the  free  acid  is  nearly 
neutralised.  Transfer  the  liquid  to  a  beaker,  wash  out  the 
bulbs,  and  complete  the  addition  of  the  soda  solution.  By 
operating  in  this  manner  there  is  less  chance  of  loss  arising 
from  incomplete  transference  of  the  acid  liquid,  and  less 
washing  water  is  afterwards  required.  It  will  be  found 
most  convenient  to  use  a  soda  solution,  of  which  about  3  c.c. 
are  equivalent  to  i  c.c.  of  normal  acid.  If  it  is  preferred  to 
determine  the  nitrogen  by  weight  (and  this  should  be  done 
if  much  empyreumatic  matter  be  present  in  the  liquid  of  the 
bulbs),  rinse  the  bulbs  into  a  beaker,  and  pour  the  solution 
through  a  moistened  filter.  Evaporate  the  filtrate  to  dryness 
with  excess  of  .platinum  tetrachloride,  transfer  the  washed 
double  salt  to  a  weighed  porcelain  crucible,  in  the  manner 
directed  on  p.  84,  and  dry  it  slowly;  heat  the  crucible 


Manures. 


337 


to  bright  redness,  and  weigh  the  residual  platinum.  194-4 
parts  of  platinum  are  equivalent  to  28  of  nitrogen. 
The  amount  of  the  nitrogen  cannot  be  calculated  from 
the  weight  of  the  double  salt,  since  it  is  apt  to  contain 
considerable  quantities  of  compounds  of  platinum  with 
organic  bases.  These  bases,  however,  contain  the  same  pro- 
portion of  nitrogen  and  platinum  as  the  ammonium-platinum 
chloride.  By  determining  the  amount  of  platinum  left  on 
ignition,  the  proportion  of  the  nitrogen  is  therefore  readily 
calculated. 

Deduct  the  amount  of  nitrogen  corresponding  to  the 
ammonia  found  in  o  :  the  difference  shows  the  quantity  of 
organic  nitrogen. 

XLIII.    BONE- DUST. 

1.  Moisture. — Dry  a  weighed  portion  at  120-130°  in  the 
air-bath. 

2.  Carbonic  Acid. — Determine  this  constituent  by  means 
of  the  apparatus  represented  in  fig.  31,  p.  86. 

3.  Fixed  Constituents. — See  p.  331. 

4.  Insoluble  Matter  and  Sand,  &c. — See  p.  332. 

5.  Soluble  Matters  after  Treatment  with  Hydrochloric  Acid. 
—See  p.  332. 

•  6.  Fat. — Treat  about  6  or  Sgrams  of  the  sample  with  boiling 
ether  in  an  apparatus  adjusted  for  distillation  per  ascensum, 
and  dry  the  insoluble  matter  at  120-130°  in  the  air-bath. 
From  the  loss  of  weight  deduct  the  moisture  found  in  i  ;  the 
remainder  gives  the  quantity  of  fatty  matter. 

7.  Gelatigenous  Matter. — Add  together  the  amounts  of  the 
several  constituents :  the  difference  required  to  make  up 
100  may  be  set  down  as  gelatigenous  substance. 

XLIV-  SUPERPHOSPHATES. 

The  phosphoric  acid  existing  in  the  majority  of  naturally 
occurring  phosphates  is  not  very  readily  dissolved  by  water, 
and  is  therefore  not  in  the  form  in  which  it  can  be  rapidly 

z 


338  Quantitative  Chemical  Analysis. 

assimilated  by  plants.  The  manure  manufacturer  converts 
a  portion  of  the  phosphoric  acid  into  the  soluble  modifica- 
tion by  treating  the  phosphorite,  bone-dust,  spent  bone- 
black,  &c.,  with  sulphuric  acid.  In  this  operation  the  in- 
soluble tricalcium  phosphate  (Ca3P2O8)  is  converted  into 
the  soluble  monocalcium  phosphate,  CaH4P2O8,  calcium 
sulphate  being  simultaneously  produced.  The  pasty  mass 
which  runs  out  of  the  apparatus  in  which  the  mixture  of 
phosphate  and  acid  is  made,  gradually  becomes  dry  on 
standing,  partly  from  the  evaporation,  and  partly  from  the 
assimilation  of  the  water.  To  increase  the  fertilising  power 
of  the  material,  or  to  satisfy  the  tastes  of  the  consumers,  the 
manufacturer  frequently  adds  various  substances,  such  as  dried 
or  liquid  blood  to  the  material  before  or  after  treatment  with 
the  acid.  Superphosphate  therefore  consists  essentially  of 
monocalcium  phosphate,  CaH4P2O8  (so-called  soluble  phos- 
phate), mixed  with  tricalcium  phosphate,  Ca3P2O8  (insoluble 
phosphate),  calcium  sulphate,  oxides  of  iron,  alumina,  mag- 
nesia, and  alkalies,  and  more  or  less  organic  matter  and 
moisture. 

Sample  the  mixture  carefully,  and  transfer  a  portion  to  a 
stoppered  bottle,  from  which  the  quantities  employed  for  the 
several  estimations  are  to  be  taken. 

1.  Water. — Dry  a  weighed  portion  of  the  sample  at  170° 
in   the  air-bath   until  it  ceases  to  lose  weight.     The  loss 
gives  the  quantity  of  moisture  and  water  existing  in  com- 
bination with  the  calcium  sulphate. 

2.  Weigh  out  10  grams  of  the  undried  superphosphate  into  a 
mortar,  add  a  small  quantity  of  cold  water,  and  triturate  with 
the  aid  of  the  pestle ;  allow  the  suspended  matter  to  settle 
for  a  few  minutes,  and  pour  the  liquid  through  a  filter  into  a 
500  c.c.  flask.     Repeat  the  extraction  with  cold  water  in  the 
same  manner  several  times  in  succession,  and  finally  wash 
the  residue  with  hot  water,  transferring  the  washings  to  the 
filter.     Dilute  the  filtrate  to  the  mark  and  shake  the  liquid. 
Weigh  the  insoluble  portion. 


Manures.  339 

I.     Examination  of  the  Filtrate. 

(a)  Estimation  of  the  Ferric  Phosphate  and  Soluble  Calcium 
Phosphate. — Transfer  200  c.c.  of  the  liquid  to  a  platinum 
dish,  and  evaporate,  adding  an  excess  of  sodium  carbonate 
and  a  little  nitre  so  soon  as  the  whole  of  the  liquid  has  been 
brought  into  the  dish.     When  the  mass  is  dry,  ignite  gently, 
mix  with  a  little  water,  and  rinse  the  contents  of  the  dish 
into  a  beaker,  add  excess  of  hydrochloric  acid,  and  heat 
until  the  liquid   is   clear.     Mix  with  excess  of  ammonia, 
acidulate  with  acetic  acid,  and  filter  off  and  weigh  the  ferric 
phosphate,  receiving  the  filtrate  in  a  250  c.c.  flask.     Dilute  to 
the  mark  and  shake.    Withdraw  successive  portions  of  50  c.c., 
and   determine  the  phosphoric  acid  by  uranium  solution. 
Transfer  100  c.c.  of  the  liquid  to  a  beaker,  and  determine 
the  lime-sxA  magnesia,  as  directed  on  p.  333. 

(b)  Estimation  of  Organic  Matter  and  Alkalies. — Evaporate 
100  c.c.  in  a  platinum  dish,  adding  milk  of  lime  until  the 
liquid  is  distinctly  alkaline.     Dry  the  residue  at  180°  and 
weigh.     Ignite  the  dried  mass  and  again  weigh ;  the  difference 
gives  the  quantity  of  organic  matter.    Boil  the  weighed  residue 
with  lime  water,  then  with  pure  water  ;  filter,  and  add  barium 
chloride  to  precipitate  the  sulphuric  acid ;  mix  with  am- 
monia, ammonium  carbonate,  and  oxalate ;  filter,  evaporate 
to  dryness  with  hydrochloric  acid,  ignite  the  residue,  treat 
with  water,  filter,  and  weigh  the  alkaline  chlorides. 

(c)  Determination  of  the  Sulphuric  Acid. — Heat  100  c.c.  of 
the  liquid  in  a  beaker,  acidulate  with  a  few  drops  of  hydro- 
chloric acid,  and  precipitate  with  barium  chloride. 

II.  Examination  of  the  Insoluble  Portion. 

(a)  Determination  of  the  Carbon. — Ignite  gently  in  a  pla- 
tinum dish  ;  the  loss  of  weight  gives  the  quantity  of  organic 
matter  and  charcoal. 

(b}  Determination  of  Sand,  Clay,  &c. — Boil  the  ignited 
portion  repeatedly  with  dilute  hydrochloric  acid,  filter  into 

z  2 


34O  Quantitative  Chemical  Analysis. 

a  |  -litre  flask,  and  wash  with  hot  water.  The  insoluble 
residue  consists  of  sand  and  clay.  Dilute  the  filtrate  to  the 
mark  and  shake. 

(c)  Determination  of  Phosphoric  Acid,   Iron,   Lime,   and 
Magnesia.  —  Transfer  100  c.c.  c/f  the  above  solution  to  a  beaker 
and  proceed  exactly  as  in  I.  (a). 

(d)  Determination  of  Sulphuric  Acid.  —  In   100  c.c.,   by 
barium  chloride  in  the  usual  manner. 

(e)  Determination  of  Total  Nitrogen.  —  In  from  i  to  2  grams 
of  the  original  substance  by  ignition  with  soda-lime.     (See 

P.  335-) 

(y~)  Determination  of  Ammonia.  —  Superphosphates   are 

occasionally  mixed  with  ammoniacal  salts.  The  amount  of 
this  ammonia  is  determined  as  in  No.  VIII.  Part  II. 

The  results  of  the  analysis  should  be  arranged  according 
to  the  subjoined  form  : 

/  Phosphoric  acid* 

Soluble  constituents      ^esia 

\  Ferric  oxide 

{Phosphoric  acidf 
Lime 
Magnesia 
Ferric  oxide 
Alumina 

Total  calcium  sulphate, 

,,     organic  matter  and  charcoal,  \ 
,,     sand  and  clay, 
,,     moisture, 

*  Equal  to      per  cent,  soluble  phosphate. 

f  Equal  to      per  cent,  insoluble  phosphate. 

I  Containing      per  cent,  of  nitrogen,  equal  to      per  cent,  ammonia. 


XLV.      ASHES  OF  PLANTS. 

The  substances  generally  present  in  estimable  quantity  in 
the  ashes  of  plants  are  silica,  phosphoric,  sulphuric,  and  car- 
bonic acids,  chlorine,  potash,  soda,  lime,  magnesia,  iron,  and 
manganese.  In  much  smaller  quantity  are  sometimes  found 


Ashes  of  Plants.  34 1 

alumina,  lithia,  strontia,  baryta,  rubidia,  copper,  fluorine, 
iodine  and  bromine,  cyanides  and  cyanates,  boracic  acid, 
sulphides,  &c. 

Certain  of  these  substances  are,  however,  never  present 
in  plants:  thus  the  cyanides  and  cyanates  are  formed  by 
the  mutual  action  of  the  carbon  and  nitrogen  in  the  plant  at 
the  high  temperature  of  the  incineration  :  in  the  case  of 
ashes  rich  in  alkalies  and  alkaline  earths  they  may  have 
been  also  formed  by  the  action  of  the  nitrogen  in  the  air 
during  the  burning.  Probably  too,  all  the  sulphur  found  in 
the  ash  did  not  originally  exist  as  sulphuric  acid  :  not 
unfrequently  sulphur  exists  in  the  unoxidised  state  in  a 
plant,  and  in  combination  with  carbon,  hydrogen,  and 
nitrogen,  forming  peculiar  organic  acids.  In  presence  of 
the  bases  the  sulphur  becomes  converted  into  sulphuric 
acid  during  the  incineration:  sometimes  a  portion  of  the 
sulphur  escapes  oxidation,  or  when  oxidised  is  again  reduced 
by  the  admixed  charcoal,  giving  rise  to  the  sulphides.  The 
main  quantity  of  the  carbonic  acid  present  in  the  ash  is 
derived  from  the  destruction  of  organic  acids  combined  with 
the  alkalies. 

In  order  to  obtain  the  ash  in  a  proper  scate  for  analysis, 
the  portions  of  the  plant  to  be  incinerated  must  be  freed 
as  far  as  possible  from  adhering  soil,  &c.,  by  brushing  or 
rubbing.  In  the  case  of  small  seeds  the  best  plan  is  to 
treat  them  in  a  beaker  with  a  small  quantity  of  water,  stir 
them  with  a  glass  rod  for  a  minute  or  so,  and  throw  them  on 
a  sieve,  the  meshes  of  which  are  sufficiently  coarse  to  allow 
the  sand  to  pass  through,  whilst  retaining  the  seeds.  Repeat 
this  operation  several  times,  but  take  care  not  to  allow  the 
seeds  to  remain  too  long  in  contact  with  the  water,  or  por- 
tions of  the  soluble  salts  will  be  dissolved  out.  Place  the 
seeds  in  a  cloth  and  rub  them  between  its  folds,  and  dry 
them  on  a  water-bath.  The  substance  to  be  incinerated  is 
weighed,  and  placed  in  a  shallow  porcelain  basin  fitting  into 
a  muffle,  which  is  to  be  gradually  heated  to  low  redness. 


342 


Quantitative  Chemical  Analysis. 


FIG.  75. 


Great  care  must  be  taken  duly  to  regulate  the  heat :  if  it  is 
too  high,  the  process  of  incineration  will  be  retarded :  the 
salts  will  fuse,  and  enclose  the  carbonaceous  matter,  thus 
protecting  it  from  the  action  of  the  air.  Moreover,  at  a  high 
temperature,  chloride  of  sodium  would  volatilise,  and  a  part 
of  the  phosphorus  would  be  lost.  It  seldom  facilitates  the 
operation  to  stir  the  heated  mass,  as  its  porosity  and  loose- 
ness of  aggregation  are  thereby  destroyed.  The  supply  of  air 
must  be  adequate,  but  not  excessive,  otherwise  particles  of 
the  ash  are  apt  to  be  carried  away  in  the  draught.  In  the 
ash  of  vegetables,  the  amount  of  alkali 
is  frequently  so  considerable  that  it  is 
almost  impossible  to  obtain  the  mass 
quite  white  at  a  temperature  sufficiently 
low  to  prevent  it  fusing.  In  this  case 
it  is  best  to  char  the  body  in  a  Hessian 
crucible  at  a  low  red  heat  (scarcely 
visible  in  daylight),  extract  the  soluble 
portion  with  water,  and  complete  the 
incineration  of  the  residue  in  a  muffle. 
In  all  cases  the  ash  must  be  weighed, 
properly  mixed  in  a  smooth  porcelain 
crucible,  and  preserved  in  a  well-stop- 
pered bottle. 

The  ash  of  organic  substances  may 
in  general  be  readily  obtained  free  from 
carbonaceous  matter  by  the  simple  ar- 
rangement seen  in  fig.  75.  The  mass 
is  charred  in  a  porcelain  dish  at  a  low 
red  heat,  and  as  soon  as  the  evolu- 
tion of  empyreumatic  matter  ceases,  the  neck  of  a  large 
retort  is  supported  over  the  dish  by  means  of  a  clamp,  and 
the  heating  is  continued  until  the  mass  is  white.  The  increased 
current  of  air  playing  over  the  heated  mass  facilitates  the 
combustion  of  the  carbon.  This  method  is  liable,  however, 
to  increase  the  amount  of  sulphates  present  in  the  ash,  owing 


Ashes  of  Plants.  343 

to  the  action  of  the  sulphuric  acid  derived  from  the  coal 
gas  :  in  cases  where  great  accuracy  is  required  the  Bunsen 
lamp  must  be  replaced  by  a  spirit  lamp. 

From  7  to  10  grams  of  the  well-mixed  and  finely-powdered 
ash  are  placed  in  a  glass  cylinder  of  about  300  cubic  centi- 
metres capacity,  provided  with  a  well-fitting  stopper.  About 
25  cubic  centimetres  of  distilled  water  are  then  added,  and 
carbon  dioxide  is  passed  into  the  cylinder.  The  delivery 
tube  of  the  apparatus  (which  must  not  dip  into  the  liquid) 
is  occasionally  withdrawn,  the  stopper  inserted,  and  the 
liquid  shaken  to  promote  the  absorption  of  the  gas.  When 
the  caustic  bases  are  completely  neutralised  and  the  solution 
saturated  (which  is  evidenced  by  the  cessation  of  the  partial 
vacuum,  and  also  by  the  bubbles  passing  upwards  between 
the  bottle  and  its  stopper  when  the  latter  is  cautiously  lifted 
after  the  liquid  has  been  shaken),  the  contents  of  the  cylinder 
are  washed  into  a  porcelain  dish,*  evaporated  to  complete 
dryness,  again  heated  with  a  small  quantity  of  water  to 
dissolve  the  alkaline  salts,  and  after  standing  a  short  time 
FIG  76  filtered  through  a  weighed  filter.  The 

filtrate  is  again  evaporated  to  dryness, 
the  saline  residue  treated  with  a  small 
quantity  of  water,  and  the  calcium  sulphate 
which  separates  out  filtered  off  through  a 
weighed  filter.  The  filtrate  is  received  in  a 
small  weighed  flask  of  150  cubic  centimetres 
capacity,  provided  with  a  side  tubulus  (fig.  76).  This  is 
easily  made  by  directing  the  flame  of  the  blowpipe  upon  the 
side  of  the  flask  until  the  glass  is  softened,  when  on  touching 
the  softened  part  by  a  thick  platinum  wire  it  will  adhere,  and  a 
portion  of  the  glass  may  be  drawn  out  in  the  form  of  a 
narrow  tube.  The  wire  is  detached  from  the  tubulus  by 
scratching  the  latter  with  a  cutting  diamond.  Care  must  be 

*  If  calcium  carbonate  crystallises  on  the  side  of  the  cylinder  it  may 
be  removed  by  adding  a  little  water,  saturating  it  with  carbonic  acid, 
and  dissolving  the  thin  crust  by  vigorously  shaking  the  liquid. 


344  Quantitative  Chemical  Analysis. 

taken  in  filtering  the  liquid  containing  the  soluble  portion 
of  the  ash,  into- this  flask  that  the  end  of  the  funnel  does  not 
dip  into  the  liquid  ;  the  funnel  must  be  maintained  in  such 
a  position  that  the  drops  in  falling  into  the  flask  are  not 
splashed  against  its  upper  sides.  The  filtrate  is  diluted  to 
about  60  cubic  centimetres,  and  well  mixed  by  shaking.  The 
edge  of  the  tubulus  is  slightly  greased,  and  the  flask  and  solu- 
tion weighed.  The  liquid  is  then  divided  in  to  six  portions,  con- 
tained in  little  beakers,  to  serve  for  the  determination  of  the 
sulphuric  acid,  alkalies,  chlorine,  phosphoric  and  carbonic 
acids,  the  sixth  portion  being  reserved  in  case  of  accident. 
The  object  of  the  tubulus  is  to  allow  of  the  liquid  being 
poured  from  the  flask  into  the  beakers  :  the  amount  taken 
for  each  determination  is  indicated  by  the  loss  of  weight 
suffered  by  the  flask  and  solution. 

The  carbonic  acid  is  determined  volumetrically  by  deci-nor- 
mal  sulphuric  acid  and  litmus  solutions  ;  the  sulphuric  acid 
and  chlorine  by  precipitation  as  barium  sulphate  and  silver 
chloride.  The  portion  for  the  phosphoric  acid  determina- 
tion is  acidified  with  hydrochloric  acid  solution,  boiled  to 
expel  carbonic  acid,  allowed  to  cool,  ammonia  added, 
together  with  a  few  drops  of  magnesia-mixture,  and  the  mag- 
nesium-ammonium phosphate  weighed  as  pyrophosphate.  In 
order  to  determine  the  amount  of  the  alkalies,  the  solution  is 
boiled  with  a  slight  excess  of  baryta  water  (best  in  a  platinum 
or  silver  dish) ;  the  sulphuric,  carbonic,  and  phosphoric  acids, 
together  with  the  greater  portion  of  the  magnesia  dissolved, 
are  thus  separated  :  the  excess  of  baryta  is  removed  by 
ammonia  and  ammonium  carbonate.  The  filtrate  is  evapo- 
rated to  dryness  in  a  platinum  dish,  gently  heated,  re-dissolved 
in  a  few  drops  of  water,  filtered  if  necessary,  a  few  drops  of 
hydrochloric  acid  added,  and  the  liquid  evaporated  to  dry- 
ness,  heated,  and  the  mixed  alkaline  chlorides  weighed. 
The  potassium  chloride  is  then  separated  by  platinum  tetra- 
chloride,.or  the  relative  amount  of  the  two  chlorides  deter- 
mined by  standard  silver.  In  cases  where  the  amount  of  the 


Ashes  of  Plants* 


345 


FIG.  77. 


soluble  portion  of  the  ash  is  comparatively  large,  more  than 
traces  of  magnesia  will  remain  in  solution  with  the  alkaline 
salts.  This  portion  of  the  magnesia  is  found  in  the  nitrate  from 
the  double  chloride  of  potassium  and  platinum  :  its  amount 
may  be  estimated  by  evaporating  the  alcoholic  solution  to 
dryness,  re-dissolving  in  water,  and 
transferring  the  liquid  to  a  small  flask 
provided  with  a  tightly-fitting  cork, 
furnished  with  two  tubes,  as  in  fig. 
77.  Hydrogen  is  led  through  the 
tube  a,  and  the  exit  tube  £,  within 
the  flask,  is  sufficiently  long  to  reach 
just  above  the  surface  of  the  liquid, 
so  as  to  ensure  the  thorough  expul- 
sion of  the  air.  When  the  flask  is 
completely  filled  with  hydrogen,  the 
ends  of  the  tubes  are  closed  by 

stoppers,  and  the  flask  is  placed  in  direct  sunlight,  when  the 
platinum  is  quickly  reduced  to  the  metallic  state,  and  the 
solution  becomes  colourless.  The  process  of  reduction 
may,  if  necessary,  be  facilitated  by  heating  the  solution  on  a 
water-bath  before  the  transmission  of  the  gas.  If  the  capa- 
city of  the  flask  is  small,  it  will  be  requisite  to  refill  it  once  or 
twice  with  hydrogen  to  ensure  the  complete  reduction  of  the 
platinum ;  it  is  then  desirable  to  displace  the  remaining  gas 
by  a  rapid  current  of  carbonic  acid,  otherwise  an  explosion 
might  occur,  particularly  if  the  contents  of  the  flask  are 
warm,  owing  to  the  surface  action  of  the  finely-divided 
platinum  on  a  mixture  of  air  and  hydrogen.  The  colourless 
solution  is  then  filtered  from  the  reduced  metal,  and,  after 
concentration,  the  magnesia  precipitated  by  sodium  phos- 
phate and  ammonia.  This  method  is  recommended  to  be 
used  in  all  accurate  separations  of  the  alkalies  from  magnesia : 
it  is  moreover  a  rapid  and  easy  mode  of  recovering  the  excess 
of  platinum  used  in  the  determination  of  potassium  or  am- 
monium salts. 


346  Quantitative  Chemical  Analysis. 

In  the  insoluble  portion  are  contained  lime,  magnesia, 
ferric  oxide  (alumina),  silica,  phosphoric,  sulphuric,  and 
carbonic  acids.  This  is  dried  at  100°  and  weighed.  It  is 
detached  as  far  as  possible  from  the  filter,  and  the  latter  in- 
cinerated. The  ash  from  the  filter-paper  is  allowed  to  fall 
into  a  porcelain  basin,  and  treated  with  water  saturated 
with  carbonic  acid,  evaporated  to  perfect  dry  ness  on  the  water- 
bath,  and  mixed  with  the  main  quantity  of  the  insoluble  por- 
tion in  a  smooth  porcelain  mortar.  The  carbonic  acid  is 
determined  in  about  i  to  2  grams  of  the  substance  according 
to  the  method  given  in  No.  V.  Part  II. ;  the  solution  in  the 
flask  serves  for  the  determination  of  the  silica,  sand,  charcoal, 
and  sulphuric  acid.  The  phosphoric  acid,  iron  (alumina),  man- 
ganese, lime,  and  magnesia  are  determined  in  about  2  grams 
of  the  remainder  of  the  insoluble  matter.  The  weighed 
portion  is  dissolved  in  nitric  acid,  and  after  separation  of  the 
silica  in  the  usual  manner,  the  solution  is  again  evaporated 
nearly  to  dryness  in  a  porcelain  basin,  and  dilute  nitric  acid 
added  until  the  bases  are  completely  dissolved,  strong  fuming 
nitric  acid  (saturated  with  the  lower  oxides  of  nitrogen) 
added  until  calcium  nitrate  begins  to  separate :  a  few 
more  drops  of  dilute  nitric  acid  are  now  added  to  destroy 
the  slight  turbidity.  The  nitric  acid  solution  of  the  sub- 
stances is  thus  in  the  highest  possible  state  of  concentration. 
It  is  covered  with  a  large  watch-glass,  gently  warmed,  and 
about  2  grams  of  tin-foil  added  in  small  portions  at  a  time. 
The  tin  is  rapidly  oxidised,  and  the  supernatant  liquid 
becomes  perfectly  clear.  The  preliminary  heating  of  the 
solution  is  absolutely  necessary,  since  in  the  cold  the  metal 
is  apt  to  become  passive,  when  it  resists  the  action  of  the 
acid.  Care  must  be  taken  to  keep  the  nitric  acid  in  sufficient 
excess,  in  order  to  prevent  the  formation  of  hydrated  mon- 
oxide, which  renders  the  solution  inconveniently  turbid. 
When  all  action  is  at  an  end,  and  the  tin  fully  oxidised,  the 
contents  of  the  dish  are  evaporated  nearly  to  dryness,  water 
is  added,  and  the  solution  filtered  The  precipitate  contains 


Ashes  of  Plants.  347 

all  the  phosphoric  acid  ;  the  bases  are  found  in  the  filtrate. 
The  precipitate,  detached  as  far  as  possible  from  the  filter,  is 
digested  in  the  smallest  possible  quantity  of  highly- concen- 
trated potash  solution  j  on  the  addition  of  water  the  solution 
will  become  perfectly  clear,  provided  no  great  excess  of  the 
alkali  has  been  used.  The  small  amount  of  the  precipitate 
still  adhering  to  the  filter  is  also  dissolved  in  a  few  drops  of 
potash  solution,  and  added  to  the  main  portion  of  the  liquid. 
The  mixture  is  then  saturated  with  sulphuretted  hydrogen, 
acetic  or  sulphuric  acid  added  in  very  slight  excess,  and  the 
precipitated  tin  sulphide  separated  by  the  filter-pump.  The 
filtrate  is  concentrated  to  a  small  bulk,  filtered  from  the 
slight  amount  of  tin  sulphide,  which  often  separates  on 
evaporation,  and  the  phosphoric  acid  precipitated  by  mag- 
nesia-mixture and  ammonia.  The  filtrate  from  the  insoluble 
tin  phosphate  is  treated  with  sulphuretted  hydrogen  to 
remove  the  lead  with  which  the  foil  is  frequently  mixed, 
filtered,  evaporated  to  a  small  bulk,  boiled,  ammonia  added 
in  slight  excess,  and  the  iron  and  alumina  filtered  off :  they 
are  separated  as  in  No.  XII.  Part  II.  The  filtrate  from  the 
precipitate  by  ammonia  contains  the  manganese,  lime,  and 
magnesia.  These  are  separated  as  in  No.  XIX.  Part  IV., 
p.  220. 


Quantitative  Chemical  Analysis. 


PART    V. 
ORGANIC   ANALYSIS. 

I.  ANALYSIS   OF    BODIES   CONTAINING   CARBON  AND  HY- 
DROGEN, OR  CARBON,  HYDROGEN,  AND  OXYGEN. 

ORGANIC  substances  containing  hydrogen,  when  heated 
with  cupric  oxide,  are  converted  into  carbon  dioxide  and 
water.  By  absorbing  the  products  of  the  combustion  in  a 
suitably-arranged  apparatus,  and  weighing  them,  we  can 
readily  calculate  the  amount  of  carbon  and  hydrogen  in 
the  substance  analysed,  from  the  knowledge  that  44  parts 
of  carbon  dioxide  contain  12  parts  of  carbon,  and  that 
1 8  parts  of  water  contain  2  parts  of  hydrogen.  If  the  sum 
of  the  amounts  of  carbon  and  hydrogen  is  equal  to  the 
weight  of  the  body  taken,  the  substance  contains  only  these 
elements  ;  if  the  body  contains  oxygen  in  addition,  the  differ- 
ence indicates  the  amount  of  this  constituent. 

Fig.  78  represents  the  apparatus  in  which  the  combustion 
may  be  conveniently  made.  The  substance  is  burnt  with 
cupric  oxide  by  the  aid  of  a  current  of  oxygen  or  air.  The 
gas-furnace  is  of  the  form  known  as  Erlenmeyer's  j  it  consists 
of  24  Bunsen-burners,  each  provided  with  a  separate  stop- 
cock worked  by  a  little  lever.  The  width  of  the  air-passages 
in  the  burners  may  be  regulated  by  a  short  piece  of  move- 
able  tube,  so  that  the  amount  of  air  passing  into  the  tube 
may  be  altered  at  will.  This  arrangement  serves  to  prevent 
the  flame  passing  down  to  the  burner  at  the  bottom  when 
the  gas-current  is  feeble.  The  tubes  end  in  a  horizontal  pipe, 
which  is  connected  with  the  gas-supply  by  wide  caoutchouc 
tubes.  The  flames  strike  against  a  semi-circular  trough  of 
well-baked  fire-clay,  resting  on  small  clay  supports ;  in  this 
trough  is  placed  the  combustion-tube.  The  side  plates  a,  a 


Organic  Analysis. 


349 


350 


Quantitative  Chemical  Analysis. 


FIG.  79. 


(fig.  79)  are  of  clay ;  they  are  moveable,  and  are  supported 
upon  a  ledge  running  the  entire  length  of  the  furnace. 

It  will  be  seen  from  their  peculiar  shape 
(fig.  79)  that  the  flames,  after  diverging 
from  beneath  the  trough,  strike  against  the 
sides ;  the  heat  is  thus  reverberated,  and 
the  tube  is  uniformly  and  regularly  heated. 
By  the  aid  of  the  clamping  screws  a  slight 
inclination  may  be  given  to  the  ledge  on 
which  the  plates  rest,  or  it  may  be  raised 
or  lowered  above  the  burners. 

The  following  articles  are  needed  to 
make  a  combustion  by  means  of  this  appa- 
ratus : — 

1.  A  Piece  of  Combustion-tube.      This 
should  be  about  4  or  5  centimetres  longer 

than  the  furnace  ;  and  it  should  be  about  2  millimetres 
thick  in  the  glass,  and  about  12  or  14  millimetres  in  internal 
diameter.  The  sharp  edges  of  the  tube  should  be  fused  in 
the  blowpipe  flame,  so  that  two  caoutchouc  stoppers,  pierced 
with  holes,  may  be  introduced  without  being  cut  or  torn. 

2.  A  Calcium  Chloride  Tube.     This  serves   to 
absorb  the  water  produced  :  it  may  conveniently 
be  arranged  as  in  fig.  80.     It  is  furnished  with 
two  bulbs,  a  and  b  :  in  the  small  neck  between 
the  bulbs   is   fused  a   piece  of   thin  glass  tube 
projecting   into   the  bulb  a.     By  carefully  regu- 
lating the  heat,  the  greater  portion  of  the  water 
produced   in   the    combustion   condenses   in   a : 
if  its  quantity  is  not  too  considerable,  it  remains 
in  this  bulb  when  the  tube  is  held  perpendicularly 
with  the   bulbs   uppermost.     After  having  been 
weighed   the  water  may  readily  be  emptied  out 
into  a  little  capsule,  and  its  purity  tested  by  its 
taste,  smell,  action  on  litmus-paper,  &c.     A  cal- 
cium chloride  tube  so  arranged  may  be  used  for  a  great 


FIG.  80. 


Organic  A  nalysis.  3  5 1 

number  of  observations  without  replenishing,  provided  that 
on  the  conclusion  of  the  experiment  the  bulb  be  emptied 
and  the  tube  dried  by  the  aid  of  a  narrow  roll  of  filter- 
paper.  To  fill  the  calcium  chloride  tube,  place  a  loose 
plug  of  cotton-wool  within  the  wide  tube,  close  the  end 
with  the  finger,  and  suck  out  the  air  at  the  narrow  end. 
On  suddenly  removing  the  finger,  the  loose  plug  is  driven 
into  the  larger  bulb  :  repeat  this  operation  until  the  long 
fibres  of  the  wool  are  within  the  neck  between  a  and  b : 
these  fibres  tend  to  prevent  the  formation  of  drops  in  the 
narrow  tube,  and  thus  to  promote  the  regularity  of  the 
passage  of  the  gas  through  the  potash  bulbs.  Fill  the 
larger  bulb  with  coarse  fragments  of  spongy  calcium  chlo- 
ride, gently  tapping  the  tube  so  as  to  shake  the  pieces 
together,  and  then  add  smaller  pieces  (not  powder)  until 
the  tube  is  nearly  filled  ;  insert  a  plug  of  cotton-wool  and 
close  the  tube  with  a  good,  softened,  tightly-fitting  cork, 
through  which  passes  a  tube  about  4  centimetres  long  and 
of  the  same  diameter  as  the  tube  of  the  potash  bulbs.  Fuse 
the  sharp  edges  of  the  tube  before  inserting  it  into  the  cork. 
After  fitting  the  cork  into  the  calcium  chloride  tube,  cut 
the  protruding  portion  with  a  sharp  knife  in  the  manner  seen 
in  fig.  80,  and  neatly  cover  the  surface  with  sealing  wax. 
Take  care  that  the  wax  is  uniformly  melted  and  is  in  a  co- 
herent piece,  otherwise  portions  are  apt  to  be  detached  in 
FIG.  81.  handling  the  tube  between  the  opera- 

tions of  weighing ;  the  experiment  may 
thus  be  nullified  or  rendered  inexact. 
The  ends  should  then  be  closed  by 
short  pieces  of  caoutchouc  tube  stopped 
with  glass  rod. 

3.  The  Potash  Bulbs.  This  appa- 
ratus serves  to  absorb  the  carbon  di- 
oxide. The  form  represented  in  fig.  81 
is  that  originally  devised  by  Liebig  (by  whom,  indeed,  the 
method  of  organic  analysis  by  combustion  with  cupric  oxide 


352  Quantitative  Chemical  A  nalysis. 

was  first  worked  out).  It  is  filled  to  the  extent  indicated  by 
the  dotted  line  in  the  figure,  with  strong  potash  solution, 
prepared  by  dissolving  3  parts  of  potash  free  from  carbonate 
in  2  parts  of  water.  The  bulbs  are  readily  filled  with  this 
liquid,  contained  in  a  porcelain  dish,  by  dipping  the  end  of 
the  tube  connected  with  the  larger  bulb  beneath  the  surface 
of  the  potash  solution  and  gently  aspirating  at  the  other  tube 
until  the  required  amount  has  been  introduced.  Carefully 
dry  the  tube,  inside  and  out,  with  paper,  and  close  the 
apparatus  by  short  caoutchouc  tubes  fitted  with  glass  rod. 
Twist  a  piece  of  platinum  wire  round  the  tubes  where 
they  touch,  in  the  manner  seen  in  the  figure :  this  serves 
to  suspend  the  bulbs  from  the  hook  of  the  balance-pan. 

The  tube  connected  with  the  smaller  bulb  is  adapted  to  a 
short  and  light  drying  tube,  c  (fig.  78),  about  5  centimetres 
long,  filled  with  soda-lime  contained  between  loose  plugs  of 
cotton-wool,  as  in  the  calcium  chloride  tube.  The  cork  is 
to  be  trimmed  and  covered  with  sealing  wax  in  the  man- 
ner already  described.  FIG.  82.  FIG.  83. 
This  apparatus  serves 
to  retain  any  carbon 
dioxide  which  may 
escape  absorption  in 
the  potash  bulbs :  it 
is  therefore  weighed 
with  the  bulbs. 

Wipe    the    potash 
bulbs  and  the  calcium  chloride  tube  with  a  soft  clean  cloth 
and  place  them  in  the  balance-case. 

Many  other  forms  of  the  potash  apparatus  have  been  de- 
scribed. Fig.  82  represents  a  modification  due  to  Geissler: 
it  will  be  seen  that  the  gas  passes  thrice  through  the 
potash  solution.  The  apparatus  requires  no  support  and  is 
readily  filled  and  emptied.  Fig.  83  shows  a  very  simple 
form  of  potash  bulbs,  originally  devised  by  Mitscherlich,  and 
modified  by  De  Koninck.  This  piece  of  apparatus  is 


Organic  Analysis.  353 

admirably  adapted  for  washing  or  drying  gases.  Carbon 
dioxide  may  also  be  absorbed  by  soda-lime,  as  we  have  fre- 
quently had  occasion  to  observe.  This  method  of  absorption 
is  especially  convenient  if  the  carbon  dioxide  is  mixed  with 
comparatively  large  quantities  of  other  gases.  In  such  a 
case  the  potash  apparatus  is  replaced  by  a  U-tube  rilled  with 
soda-lime  and  calcium  chloride,  as  described  on  p.  87. 

4.  A   Platinum  Boat,  to   contain   the   substance  to  be 
analysed.     This  should  be  of  such  size   as  to  pass  readily 
into  the  tube.     It  may  conveniently  be  70  mm.  long  and 
8  mm.  deep. 

5.  Cupric  Oxide.     Strongly  heat  some  clean  copper  scales 
in  a  muffle,  and  when  they  are  sufficiently  cool,  transfer  them 
to  a  porcelain  basin  and  heat  them  with  nitric  acid  (sp.  gr. 
1-2).     Evaporate  the  pasty  mass  to  dryness  on  a  sand-bath, 
pound   it  up  and  heat  it   strongly  in  a   covered   Hessian 
crucible.     Break  the  crucible,  carefully  remove  any  pieces  of 
clay,  and  coarsely  powder  the  fused  cupric  oxide,  pass  the 
powder  through  a  sieve  of  wire  gauze,  to  separate  the  fine 
portions.    The  cupric  oxide  to  be  used  in  the  analysis  should 
be  in  little  pieces  about  the  size  of  hemp- seed.  The  finer  por- 
tion should  be  preserved  in  a  stoppered  bottle  :  it  is  useful 
for  the  determination  of  nitrogen,  as  described  hereafter. 

6.  Copper  Gauze  and  Wire.     Roll  two  pieces  of  fine  wire 
gauze,  about  2  centimetres  broad,  into  plugs  of  a  size  just 
sufficient  to  pass  easily,  but  with  a  little  friction,  into  the 
combustion-tube.     Heat  them  in  the  Bunsen  flame  to  re- 
move any  adhering  greasy  matter,  and  when  cold  push  one 
of  them  down  about  25  centimetres  into  the  combustion- 
tube,  and  fill  up  the   tube   from   the  other  end  with   the 
coarsely-powdered  cupric  oxide,  occasionally  tapping  it  so 
as  to  shake  the  pieces  as  closely  together  as  possible.    When 
the  tube  is  filled  to  within  6  centimetres  of  the  end,  insert 
the  second  plug  of  metallic  copper.     The  layer  of  copper 
oxide  should  be  about  54  centimetres  in  length  :  there  is  no 
necessity  to  leave  a  channel  above  it  for  the  gases,  since  from 

A  A 


354  Quantitative  Chemical  Analysis. 

the  coarseness  of  the  powder  there  is  ample  room  for  their 
escape.  Over  the  end  of  the  tube  is  placed  a  small  circular 
disc  of  copper  (fig.  78),  readily  moveable  along  the  tube. 
This  serves  to  protect  the  caoutchouc  stopper  and  to  shield 
the  little  bulb  a  of  the  calcium  chloride  tube  from  the  heat : 
by  moving  it  backwards  or  forwards  along  the  tube,  as  occa- 
sion requires,  the  condensation  within  the  tube  of  the  water 
produced  in  the  combustion  may  be  entirely  prevented. 

Cut  another  piece  of  the  copper  gauze,  about  10  centi- 
metres broad,  and  of  the  same  length  as  you  have  found 
suitable  for  the  plugs,  and  roll  it  round  a  piece  of  stout 
copper  wire,  about  12  centimetres  long;  the  one  end  of  the 
wire  should  be  bent  sharply  upon  itself  so  as  to  hold  the 
copper  gauze  firmly  near  one  corner :  the  gauze  is  then 
turned  over  the  wire  along  its  entire  length  and  wrapped 
round  so  as  to  form  a  cylinder,  which  easily  passes  into  the 
combustion-tube.  The  other  end  of  the  wire  should  be 
bent  so  as  to  form  a  little  ring  of  less  diameter  than  the  tube. 
In  the  combustion  this  long  cylinder  of  gauze  is  placed 
behind  the  platinum  boat  containing  the  substance  to  be 
analysed.  The  vacant  space  in  the  tube,  that  is,  the  portion 
before  the  first  copper  plug,  should  be  sufficiently  large  to 
hold  the  platinum  boat  and  copper  cylinder,  and  still  leave 
room  for  the  insertion  of  the  cork. 

7.  An  Apparatus  for  drying  and  removing  Carbon  Dioxide 
from  the  Air  and  from  Oxygen. — This  may  be  arranged  as  in 
fig.  78  :  the  lower  neck  of  the  cylinder  is  partially  closed  by  a 
few  fragments  of  glass,  and  the  cylinder  is  half  filled  with 
soda-lime  in  coarse  fragments  :  over  this  is  placed  a  layer  of 
cotton-wool,  and  the  remainder  is  filled  with  calcium  chloride 
in  loose  spongy  pieces.  The  cylinder  is  closed  by  a  caout- 
chouc stopper  carrying  a  bent  tube  and  leading  to  the  two  large 
U-tubes  (a)  and  (a),  also  filled  with  calcium  chloride.  Before 
entering  the  cylinder,  the  air  or  oxygen  traverses  the  wash- 
bottle  bt  containing  strong  solution  of  caustic  potash.  This 
removes  the  greater  portion  of  any  accompanying  carbon 


Organic  A  nalysis.  355 

dioxide,  and  also  serves  to  indicate  the  speed  with  which 
the  gas  passes  into  the  combustion-tube. 

8.  A  Bell-jar  fitted  with  a  Cork  and  Calcium  Chloride  Tube. 
The  bell-jar  stands  in  a  vessel  containing  water.  By  con< 
necting  it  with  the  potash  apparatus  in  the  manner  seen  in 
fig.  78  the  pressure  within  the  combustion-tube  is  decreased 
in  proportion  to  the  height  of  the  column  of  water  within  the 
bell-jar  above  the  level  of  that  in  the  trough. 

The  Process. — When  everything  is  arranged,  gently  heat  the 
combustion-tube,  having  previously  removed  the  platinum 
boat,  and  pass  a  slow  current  of  dry  air  over  the  copper 
oxide  to  expel  any  hygroscopic  moisture.  Whilst  the  oxide 
is  being  heated,  weigh  the  potash  bulbs  and  calcium  chloride 
tube  without  their  caoutchouc  stoppers',  when  you  have  de- 
termined their  weight,  replace  the  stoppers.  In  about  10  or 
15  minutes  turn  down  the  flames  beneath  the  tube  and  allow 
the  oxide  to  cool  in  the  current  of  dry  air.  We  will  assume, 
by  way  of  example,  that  you  are  about  to  analyse  pure  cane- 
sugar.  This  should  be  previously  powdered  and  dried  in 
the  steam-bath.  Heat  the  platinum  boat  to  redness  and 
allow  it  to  cool  in  the  desiccator.  Weigh  it  and  transfer 
about  0*4  grm.  of  the  sugar  to  the  boat,  and  again  weigh  : 
the  increase  in  the  weight  of  the  boat  shows  the  amount 
taken  for  analysis.  The  sugar  may  be  accurately  weighed 
in  this  manner,  as  it  is  not  hygroscopic.  Stop  the  air- 
current,  and  adapt  the  weighed  calcium  chloride  tube  to  the 
combustion-tube  by  means  of  the  caoutchouc  stopper,  and 
connect  the  potash  bulbs,  by  a  piece  of  well-fitting  caoutchouc 
tubing,  with  the  calcium  chloride  tube  in  the  manner  seen  in 
fig.  78.  There  is  no  necessity  to  bind  the  caoutchouc  to 
the  glass  tubes,  since  the  reduced  pressure  within  the  appa- 
ratus, caused  by  the  column  of  water  in  the  bell-jar,  effectu- 
ally prevents  leakage.  Partially  fill  the  bell-jar  with  water 
by  aspirating  at  the  end  of  the  tube,  turn  the  stopcock 
so  as  to  prevent  the  entrance  of  the  air,  and  connect 
the  caoutchouc  tubing  with  the  end  of  the  soda-lime 

A  A  2 


35^  Quantitative  Chemical  Analysis. 

tube  of  the  potash  apparatus  (fig.  78).  Remove  the  stopper 
at  the  further  end  of  the  combustion-tube,  withdraw  the 
cylinder  of  copper  gauze  (which  will  now  be  superficially 
oxidised),  insert  the  platinum  boat  containing  the  weighed 
amount  of  the  sugar,  and  replace  the  cylinder,  pushing  the 
boat  nearly  to  the  plug  of  copper  gauze.  Again  -fit  in  the 
cork  and  connect  the  caoutchouc  tube  of  the  wash-bottle  (b) 
with  the  gasometer  containing  the  oxygen.  Now  cautiously 
open  the  stopcock  of  the  bell-jar  and  incline  the  potash 
bulbs  in  the  manner  seen  in  the  figure :  the  smaller  bulb 
should  be  about  half  filled  with  the  potash  solution.  Light 
the  first  6  or  8  burners  (beginning  at  the  end  nearest  the 
calcium  chloride  tube),  and  gradually  heat  the  tube.  As  it 
becomes  red-hot,  light  successive  burners  until  it  is  at  a  dull 
red  heat.  Now  light  the  last  two  or  three  burners  at  the 
other  end  of  the  tube,  immediately  under  the  gauze  cylinder, 
so  as  to  heat  it  gently,  and  turn  on  a  slow  stream  of  oxygen 
(about  a  bubble  every  two  seconds  suffices  at  the  commence- 
ment of  the  process).  Continue  to  ignite  successive  burners 
so  as  to  heat  fresh  portions  of  the  copper  oxide  :  when  the 
tube  is  at  a  dull  red  heat  to  within  4  or  5  cm.  of  the 
platinum  boat,  turn  on  the  gas  in  one  of  the  burners  imme- 
diately underneath  the  boat,  and  gently  heat  it.  The  sugar 
will  quickly  melt,  become  brown,  and  give  off  vapours. 
Carefully  observe  the  movements  of  the  liquid  in  the  potash 
bulbs,  and  regulate  the  heat  so  as  to  preserve  a  uniform 
passage  of  gas  into  the  bulbs. 

As  soon  as  the  sugar  in  the  boat  appears  to  be  completely 
charred,  and  the  amount  of  carbon  dioxide  passing  into  the 
bulbs  becomes  small,  increase  the  heat  beneath  the  boat  (by 
this  time  the  whole  of  the  burners  should  be  lighted),  and 
send  a  slightly  brisker  current  of  the  dry  oxygen  (about  one 
bubble  per  second)  through  the  apparatus.  The  carbo- 
naceous matter  within  the  boat  gradually  burns  :  as  soon  as 
it  has  disappeared  gradually  diminish  the  flames  underneath 
the  gauze  cylinder  and  platinum  boat,  turn  on  a  little  more 


Organ  ic  A  nalysis.  357 

oxygen,  and  when  the  gas  appears  to  pass  unabsorbed  through 
the  potash  bulbs  gradually  lower  the  flames  along  the  entire 
length  of  the  tube.  Close  the  caoutchouc  tube  of  the 
wash-bottle,  and  transfer  it  to  the  gasometer  of  air,  and  send 
a  current  of  air  through  the  tube  to  displace  the  oxygen. 
In  a  few  minutes  disconnect  the  potash  bulbs  and  calcium 
chloride  tube  (taking  care  to  hold  the  latter  so  that  the 
water  condensed  in  the  smaller  bulb  does  not  flow  out),  fit 
in  their  respective  stoppers,  wipe  them,  re-weigh  them  (of 
course  without  the  stoppers).  Allow  the  combustion-tube  to 
cool  gradually:  if  care  be  taken  to  anneal  it  properly  it 
will  serve  a  great  number  of  times  without  rearrangement. 
The  heat  need  not  be  so  high  as  to  distort  the  tube  :  the 
great  majority  of  carbonaceous  substances,  especially  if  they 
contain  oxygen,  burn  with  comparative  ease  in  contact  with 
copper  oxide  and  free  oxygen.  The  apparatus  is  ready  for 
a  second  combustion  :  if  the  analysis  of  the  sugar  has  to  be 
repeated  it  is  of  course  not  necessary  to  wait  until  the  copper 
oxide  and  tube  are  completely  cold. 

In  the  analysis  of  volatile  liquids  the  substance  is  weighed 
out  in  little  bulbs  of  the  shape  seen  in  fig.  84.     These  are 

made  from  tube,  obtained  by  drawing 
______     out  a  piece  of  wide  glass  tubing  before 

the  blowpipe  until  it  is  about  5  milli- 
metres in  external  diameter.  The  tube  to  contain  the  liquid 
should  be  about  30  millimetres  in  length  in  the  wider  portion  : 
the  narrow  portion  should  be  short  enough  to  allow  the 
tube  to  rest  in  the  platinum  boat.  The  tube  is  weighed  and 
passed  once  or  twice  through  the  Bunsen  flame,  and  whilst 
still  hot  the  open  end  is  plunged  beneath  the  surface  of  the 
liquid  to  be  analysed.  As  the  tube  cools  the  liquid  is  driven 
into  it  to  replace  the  air  expelled  on  warming.  Withdraw 
the  tube  from  the  liquid  and  cause  the  small  portion  within 
the  bulb  to  boil  briskly  so  as  to  drive  out  all  the  air,  and 
again  plunge  the  end  of  the  tube  into  the  liquid.  The  bulb 
will  now  be  almost  completely  filled.  Except  in  the  case  of 


358  Quantitative  Chemical  Analysis. 

very  volatile  liquids  it  is  unnecessary  to  seal  the  end  of  the 
capillary  tube  :  the  bulb  containing  the  liquid  may  be  accu- 
rately weighed  and  transferred  to  the  combustion-tube  without 
any  appreciable  loss  from  evaporation.  The  process  of 
combustion  does  not  differ  in  any  essential  particulars  from 
that  already  described.  It  is  advisable,  however,  to  expel 
the  liquid  from  the  bulb  before  the  copper  oxide  is  heated 
in  its  vicinity.  As  soon  as  the  copper  oxide  is  red  hot  to 
within  15  centimetres  of  the  end  of  the  platinum  boat, 
remove  one  of  the  heated  clay  plates,  and  place  it  immedi- 
ately over  the  boat  :  if  the  liquid  is  moderately  volatile  it 
will  be  readily  and  gradually  expelled.  If  not,  the  bulb  must 
be  heated  by  a  very  small  flame.  The  combustion  of  volatile 
liquids  demands  great  care  and  attention ;  the  operation  must 
not  be  hurried,  or  portions  will  escape  unburnt. 

II.   ANALYSIS  OF  ORGANIC  SUBSTANCES  CONTAINING 
NITROGEN. 

The  determination  of  the  several  elements  contained 
in  an  azotised  organic  compound  cannot  be  very  con- 
veniently made  in  a  single  operation.  It  is  usually  pre- 
ferred to  estimate  the  carbon  and  hydrogen  in  one  portion, 
and  to  determine  the  nitrogen  in  a  second  quantity.  Nitro- 
genous organic  substances  when  burnt  with  copper  oxide, 
particularly  if  free  oxygen  be  present,  are  apt  to  evolve 
nitroxygen  compounds,  which  condense  in  the  calcium 
chloride  tube  and  potash  bulbs,  and  vitiate  the  results  of  the 
carbon  and  hydrogen  determinations.  By  passing  the  mixed 
products  of  combustion  over  heated  metallic  copper  the 
nitroxygen  compounds  are  decomposed;  the  oxygen  combines 
with  the  copper,  and  the  nitrogen  passes  unabsorbed  through 
the  apparatus.  In  the  combustion  of  organic  substances 
containing  nitrogen  it  is  necessary,  therefore,  to  introduce  a 
cylinder  of  copper  gauze,  about  12  centimetres  long,  rolled 
on  a  stout  copper  wire,  exactly  like  that  placed  in  the 
posterior  part  of  the  tube  behind  the  platinum  boat.  This 


Determination  of  Nitrogen.  359 

is  to  be  kept  at  a  bright  red  heat  during  the  operation  :  the 
carbon  and  hydrogen  may  then  be  determined  accurately  in 
the  ordinary  way ;  or  a  length  of  from  four  to  six  inches  of 
a  mixture  of  potassium  dichromate  and  manganic  oxide  is 
placed  in  the  fore  part  of  the  tube  and  kept  at  a  temperature  of 
about  250°.  This  mixture  readily  absorbs  nitroxygen  fumes. 

Determination  of  Nitrogen  by  Volume.  Maxwell  Simpson's 
Method. — This  process  is  applicable  to  all  nitrogenous  bodies, 
inorganic  and  organic.  The  substance  is  burnt  by  a  mixture 
of  cupric  and  mercuric  oxides  in  a  tube  from  which  the  air 
has  previously  been  expelled  by  a  current  of  carbon  dioxide  : 
the  nitrogen  and  carbon  dioxide,  together  with  the  excess  of 
free  oxygen,  are  passed  over  strongly-heated  metallic  copper, 
which  retains  the  latter  gas :  the  remaining  gases  are  col- 
lected in  an  apparatus  standing  over  mercury  and  partially 
filled  with  strong  solution  of  caustic  potash,  which  absorbs 
the  carbon  dioxide  :  the  residual  nitrogen  is  transferred  to  a 
measuring  tube  standing  over  mercury,  and  its  volume  is 
accurately  determined.  From  the  known  weight  of  a  litre 
of  nitrogen  the  weight  of  the  gas  is  readily  calculated. 

A  piece  of  strong  combustion-tube  about  80  centimetres 
long  is  sealed  and  rounded  at  one  end  like  a  test-tube.  A 
mixture  of  1 2  grams  of  manganous  carbonate  or  magnesite 
dried  at  100°,  and  2  grams  of  precipitated  mercuric  oxide  are 
introduced  into  the  tube.  Insert  a  plug  of  recently-ignited 
asbestos,  pushing  it  down  to  within  2  centimetres  from  the 
mixture,  and  afterwards  add  about  i  gram  of  the  mercuric 
oxide.  Weigh  out  about  0*6  gram  of  the  nitrogenous  substance 
to  be  analysed  into  a  glazed  porcelain  mortar,  and  mix  it  with 
about  45  times  its  weight  of  a  previously-prepared  mixture 
of  4  parts  of  finely-powdered  and  recently-ignited  cupric 
oxide  and  5  parts  of  the  dried  mercuric  oxide.  Transfer  the 
mixture  to  the  tube  without  loss,  and  rinse  the  mortar  with 
a  fresh  portion  of  the  two  oxides,  adding  the  rinsings  to  the 
tube.  Push  down  a  second  and  thick  plug  of  asbestos  to 


360 


Quantitative  Chemical  Analysis. 


FIG.  85. 


within  about  30  centimetres  from  the  first,  and  then  a  layer, 
about  9  centimetres  in  length,  of  pure  cupric  oxide  ;  next  a 
third  asbestos  plug,  and  lastly  a  layer  not  less  than  20  centi- 
metres long  of  metallic  copper,  prepared  by  reducing  gran- 
ular cupric  oxide  in  a  stream  of  carbon  monoxide.  Draw 
out  the  end  of  the  combustion-tube  before  the  blowpipe  and 
connect  it  with  the  bent  delivery  tube  #,  which  dips  be- 
neath the  surface  of 
the  mercury  in  the 
trough  (fig.  85).  Be- 
fore placing  the  tube 
in  the  furnace  tap  it 
gently  on  the  table 
to  shake  down  the 
several  layers,  in  or- 
der to  leave  a  chan- 
nel for  the  escaping 
gases. 

The  vessel  b  has  a 
capacity  of  about  200 
c.c.  It  is  provided 
with  a  glass  stopcock 
and  bent  delivery  tube 
as  represented  in  fig.  85.  To  ascertain  if  the  stopcock  is  per- 
fectly air-tight,  fill  the  apparatus  completely  with  mercury 
and  place  it  on  its  foot :  any  leakage  will  immediately  reveal 
itself  by  the  mercury  flowing  out  of  the  tubulus.  If  the  stop  - 
cock  is  found  to  be  tight  replace  about  20  c.c.  of  the 
mercury  with  a  strong  solution  of  caustic  potash,  and  place 
the  vessel  in  the  mercurial  trough  with  its  tubulus  beneath 
the  surface  of  the  metal. 

Heat  a  portion,  say  the  posterior  half  of  the  manganous 
carbonate  or  magnesite,  and  drive  out  the  air  within  the 
tube  by  a  brisk  current  of  carbon  dioxide.  At  the  same 
,time  commence  to  heat  the  portion  of  the  tube  occupied  by 
the  metallic  copper  and  pure  cupric  oxide.  As  soon  as  the 
escaping  gas  is  free  from  air  (which  is  readily  ascertained  by 


Determination  of  Nitrogen. 


361 


allowing  a  quantity  to  pass  into  a  test-tube  filled  with  potash 
solution,  when  no  bubble  should  be  left),  and  the  anterior 
portion  of  the  tube  is  well  heated,  insert  the  end  of  the 
delivery  tube  through  the  tubulus  of  the  vessel,  and  gra- 
dually heat  successive  portions  of  the  tube  occupied  by  the 
mixture  of  nitrogenous  substance,  cupric  and  mercuric  oxides, 
beginning  with  the  part  nearest  to  the  pure  cupric  oxide- 
As  soon  as  no  further  evolution  of  gas  is  observed,  and  the 
whole  length  of  the  tube  (with  the  exception  of  the  part 
occupied  by  the  undecomposed  magnesite  or  manganous 
carbonate)  is  at  a  bright  red  heat,  heat  the  remainder,  so  as 
to  cause  a  rapid  evolution  of  carbon  dioxide,  by  which  the 


FIG.  86. 


nitrogen  still  existing  in  the  tube  is  expelled.  Withdraw  the 
delivery  tube  from  the  tubulus,  and  allow  the  gas  in  b  to  re- 
main over  the  caustic  potash  solution  for  about  an  hour  to  ab  • 
sorb  the  last  traces  of  the  carbon  dioxide.  The  pure  nitrogen 
has  now  to  be  transferred  to  a  measuring  tube  in  order  that  its 
volume  may  be  determined.  The  bent  tube  c,  fig.  86,  which 
is  contracted  at  d,  is  fitted  by  the  aid  of  a  caoutchouc 
stopper  into  the  tubulus,  underneath  the  surface  of  the  mer- 
cury in  the  trough.  To  prevent  the  possibility  of  any  air 


362 


Quantitative  Chemical  Analysis. 


FIG.  87. 


adhering  to  the  stopper  and  so  finding  its  way  into  the 
nitrogen,  it  is  advisable  to  moisten  the  stopper  with  a  solu- 
tion of  corrosive  sublimate  before  inserting  it  into  the 
tubulus.  Fill  up  the  bent  tube  with  mercury  and  remove  b 
from  the  trough.  Place  a  drop  of  water  in  the  measuring  tube, 
fill  it  with  mercury,  and  invert  it  beneath  the  surface  of  the 
metal  in  the  trough.  Place  the  end  of  the  delivery  tube  e 
beneath  the  measuring  tube,  cautiously  turn  the  stopcock, 
and  allow  the  gas  to  escape  from  b.  When  the  level  of  the 
mercury  in  c  approaches  the  contracted  portion,  close  the 
stopcock,  refill  the  tube  with  the  metal, 
and  reopen  the  stopcock,  and  so  gra- 
dually transfer  the  nitrogen  into  the 
measuring  tube,  closing  the  stopcock  as 
soon  as  the  potash  solution  touches  it. 
Of  course  the  delivery  tube  is  thus  left 
filled  with  nitrogen,  but  as  an  identical 
volume  of  air  (viz.  that  which  originally 
filled  it)  has  been  transferred  to  the 
measuring  tube,  no  error  is  committed. 
Read  off  the  volume  of  the  moist  gas 
and  correct  it  for  pressure,  tension  of 
aqueous  vapour,  and  temperature,  and 
calculate  the  weight  of  the  nitrogen  :  a 
litre  of  nitrogen  under  the  standard 
conditions  of  temperature  and  pressure 
weighs  i  -2  5  5  gram. 

The  above  method  of  measuring  the 
volume  of  the  nitrogen  may  be  much 
simplified  by  the  use  of  the  apparatus 
devised  by  Hugo  Schiff,*  and  repre- 
sented in  fig.  87.  The  burette  A,  which  is  fitted  with  a  glass 
stopcock,  c,  contains  about  120  c.c.  down  to  the  side  tube  a, 
and  stands  in  a  wooden  foot,  which  may  be  rendered  more 


Fresenius,  Zeitschrift  fiir  anal.  Chemie,  p.  430.      1868 


Determination  of  Nitrogen.  363 

stable  by  being  weighted  with  lead.  At  about  2  centimetres 
beneath  the  side  tube  a,  is  a  second  tubulus,  ^,  inclined 
upwards  in  the  manner  seen  in  the  figure.  Through  this  tube 
is  poured  mercury  to  a  height  of  2  or  3  millimetres  above 
the  lower  opening.  The  vessel  B,  holding  from  150  to  170 
c.c.,  is  supported  by  a  metallic  ring  attached  to  the  clamp  e : 
and  may  thus  be  readily  placed  at  any  desired  height  along 
the  burette  :  B  is  connected  by  a  strong  caoutchouc  tubing, 
previously  soaked  in  melted  paraffin,  with  the  side  tube  a. 
B  is  filled  with  a  strong  solution  of  potassium  hydrate  Of  sp. 
gr.  1-5,  prepared  by  dissolving  potash  in  an  equal  weight  of 
water :  its  neck  is  closed  by  a  cork,  in  which  a  narrow 
opening  is  cut.  On  closing  the  tubulus  b  with  a  cork,  and 
on  opening  the  stopcock  and  raising  B,  the  potash  solution 
flows  over  into  the  burette  and  completely  fills  it.  The 
stopcock  is  now  closed  and  the  vessel  B  is  lowered  nearly 
to  the  foot  of  the  burette  :  the  stopper  may  then  be  with- 
drawn from  b  without  the  mercury  being  forced  out. 

The  delivery  tube  of  the  combustion-tube  is  then  pushed 
through  b  as  soon  as  all  the  air  has  been  expelled.  The 
volume  of  the  nitrogen  is  then  directly  measured,  the  vessel 
B  being  raised  until  the  levels  of  the  potash  solution  in 
both  pieces  of  the  apparatus  are  coincident.  The  nitrogen 
may  without  sensible  error  be  assumed  to  be  dry:  the 
amount  of  moisture  present  in  it  is  probably  never  more 
than  o'oo7  of  its  volume.  This  additive  quantity  serves 
in  some  measure  to  compensate  for  the  deficit  in  the 
amount  of  nitrogen  obtained,  due  to  the  impossibility  of 
entirely  preventing  the  formation  of  nitroxygen  compounds 
in  the  process  of  combustion. 

Estimation  of  Nitrogen  as  Ammonia. — By  Burning  with 
Soda-lime. — This  process,  which  is  applicable  to  all  nitro- 
genous substances  excepting  the  so-called  nitro-compounds, 
e.g.  nitre-benzol,  amyl  nitrate,  &c.,  has  already  been  described 
on  P-  334- 


364  Quantitative  Chemical  Analysis. 

III.     ANALYSIS  OF   ORGANIC  SUBSTANCES   CONTAINING 
CHLORINE,  BROMINE,  AND  IODINE. 

When  an  organic  compound  containing  a  halogen,  chlorine, 
for  example,  is  burnt  with  cupric  oxide,  cuprous  chloride  is 
formed,  which,  being  volatile,  is  carried  forward  in  the 
stream  of  gas  and  condenses  in  the  calcium  chloride  tube, 
and  thus  renders  the  determination  of  the  hydrogen  in- 
exact. If  the  gases  within  the  tube  contain  free  oxygen 
the  cuprous  chloride  is  more  or  less  decomposed,  cupric 
oxide  being  formed,  and  chlorine  eliminated.  This  is 
retained  partly  by  the  calcium  chloride,  partly  by  the 
potash  solution.  By  inserting  a  cylinder  of  copper  gauze 
in  the  anterior  portion  of  the  tube,  the  chlorine  may  be 
arrested  so  long  as  the  amount  of  oxygen  is  not  sufficient  to 
oxidise  the  copper.  By  mixing  the  cupric  oxide  with  a 
small  quantity  of  lead  oxide  the  chlorine  may  be  entirely 
retained. 

The  determination  of  the  carbon  and  hydrogen  in  com- 
pounds containing  chlorine  is  best  effected  by  heating  with 
lead  chromate.  This  substance  is  readily  made  by  mixing 
potassium  chromate  and  lead  nitrate  or  acetate  solutions, 
thoroughly  washing  the  dense  yellow  precipitate,  drying  it, 
heating  it  to  redness  in  a  covered  clay  crucible,  and  coarsely 
powdering  it.  The  combustion  is  made  in  the  manner 
already  described  in  the  case  of  copper  oxide. 

Determination  of  the  Halogen. — A  narrow  piece  of  com- 
bustion-tube about  40  centimetres  long  is  sealed  and  rounded 
at  the  end  like  a  test-tube.  A  small  quantity  of  coarsely- 
powdered  and  recently-burnt  lime  is  introduced  into  it,  so  as 
to  occupy  a  length  of  4  centimetres.  The  compound  to  be 
analysed,  if  solid,  is  weighed  out  into  the  combustion-tube, 
and  mixed  with  a  quan-  FlG  88 

tity  of  the  lime  in  mo- 
derately  fine  powder, 
by  the  aid  of  a  brass  wire  bent  in  the  manner  seen  in  fig.  88. 


Determination  of  the  Halogen.  365 

By  twisting  this  wire  among  the  fragments  the  substance 
and  the  lime  are  uniformly  mixed.  The  wire  is  rinsed 
from  any  adhering  powder  by  a  further  quantity  of  lime,  and 
the  tube  is  filled  with  the  coarsely-powdered  lime  to  within 
about  3  or  4  centimetres  from  the  open  end,  placed  in  the 
furnace  and  closed  by  a  cork  carrying  a  short  piece  of  bent 
tube,  which  dips  beneath  the  surface  of  water  contained  in 
a  small  beaker.  This  serves  to  maintain  a  slight  pressure 
within  the  tube  and  tends  to  prevent  the  escape  of  any  of 
the  halogen.  Commence  the  operation  by  heating  the  an- 
terior portion  of  the  tube,  and  gradually  approach  the  part 
containing  the  substance  as  the  lime  becomes  red-hot. 
Having  lighted  all  the  burners  beneath  it,  continue  to  heat 
the  tube  until  the  cessation  of  gas  bubbling  through  the 
water  tells  you  that  the  process  is  finished.  When  the  tube 
is  cold,  empty  the  loose  fragments  of  the  lime  into  about 
150  c.c.  of  water,  and  half  fill  the  tube  with  water  to  dissolve 
any  fused  substance  adhering  to  the  glass.  Acidify  the  liquid 
with  moderately  dilute  nitric  acid  free  from  chlorine :  an 
excess  of  nitric  acid  is  readily  indicated  by  the  change  in 
the  colour  of  the  suspended  carbonaceous  matter.  Immedi- 
ately all  the  lime  is  dissolved  the  precipitate  becomes  quite 
black.  The  liquid  is  filtered  and  treated  with  silver  nitrate 
solution,  and  the  precipitated  silver  salt  washed,  dried,  and 
weighed.  Of  course  the  quantity  of  the  chlorine  may  be 
estimated  volumetrically  by  standard  silver  solution  and 
potassium  chromate  if  care  be  taken  to  neutralise  the  excess 
of  nitric  acid  by  well- washed  precipitated  calcium  carbonate, 
or  by  the  addition  of  sodium  carbonate  solution. 

Liquids  containing  chlorine,  &c.,  are  weighed  out  in  bulbs, 
as  described  on  p.  354  :  after  the  introduction  of  a  layer  of 
lime,  about  4  centimetres  long,  the  bulb  is  allowed  to  slide 
down  the  tube,  which  is  then  immediately  filled  up  with  lime. 
When  about  half  the  length  of  the  tube  has  been  heated,  ex- 
pel the  liquid  from  the  Ivlb  by  gently  heating  the  tube  where 


366  Quantitative  Chemical  Analysis. 

it  is  situated,  and  conduct  the  remainder  of  the  operation  as 
described. 

Many  organic  substances  containing  a  halogen  may  be 
very  conveniently  analysed  by  digesting  them  with  water  and 
sodium  amalgam.  The  liquid  poured  off  the  residual  mer- 
cury is  acidified  with  nitric  acid  and  the  chlorine  determined 
in  the  usual  manner. 

Certain  organic  iodides  are  decomposed  by  heating  them 
with  an  alcoholic  solution  of  silver  nitrate :  the  silver  iodide 
thus  formed  may  be  filtered  off,  dried,  and  weighed. 

IV.    ANALYSIS  OF  ORGANIC  SUBSTANCES  CONTAINING 
SULPHUR  AND  PHOSPHORUS. 

The  combustion  of  organic  bodies  containing  sulphur  is 
most  accurately  made  with  lead  chromate:  the  only  pre- 
caution needed  is  to  maintain  the  anterior  portion  of  the 
tube,  to  the  extent  of  15  or  20  centimetres,  at  a  very  low  red 
heat  only.  Under  these  circumstances  no  sulphur  dioxide 
passes  into  the  absorption  apparatus, 

Determination  of  Sulphur. — Solid  substances  containing 
sulphur  may  be  decomposed  by  fusion  with  potassium  hydrate 
and  pure  nitre.  Place  a  quantity  of  potassium  hydrate  in  a 
silver  dish,  mix  it  with  about  J  of  its  weight  of  nitre  and  fuse 
the  mixture.  Allow  it  to  cool  and  add  to  it  the  weighed 
quantity  of  the  sulphur  compound.  Heat  gently,  and  stir 
continually  with  a  silver  spatula,  adding  little  by  little  a  small 
quantity  of  nitre  if  the  carbon  appears  to  be  but  slowly  con- 
sumed. When  the  mass  is  cold,  dissolve  it  in  water,  acidify 
with  hydrochloric  acid,  boil,  and  add  barium  chloride. 
Treat  the  precipitated  barium  sulphate  in  the  manner  de- 
scribed on  p.  169. 

Solid  compounds  of  sulphur  may  also  be  analysed  by 
digesting  them  with  strong  potash  solution  contained  in  a 
large  porcelain  crucible,  and  passing  a  stream  of  chlorine  into 


Sulphur  and  Pliosphorus.  367 

the  liquid  until  the  substance  is  completely  decomposed. 
Acidify,  heat  gently  to  expel  excess  of  chlorine,  filter,  and 
add  barium  chloride. 

Camus'  Method.  Applicable  to  the  Estimation  of  Sulphur 
and  Phosphorus  in  Solid  and  Liquid  Substances. — The  com- 
pound is  oxidised  by  the  action  of  nitric  acid  of  sp.  gr. 
1-2.  From  0-2  gram  to  0-4  gram  of  the  substance  is 
weighed  out  in  a  thin  glass-bulb,  care  being  taken 
that  but  little  air  is  enclosed  within  the  bulb.  The 
sealed  bulb  is  brought  into  a  tube  of  hard  glass  of  about 
10  or  12  millimetres  in  internal  diameter,  sealed  and  rounded 
at  one  end  like  a  test-tube,  together  with  from  20  to  60  times 
its  weight  of  nitric  acid  of  sp.  gr.  1-2.  The  tube  must  not 
be  more  than  half  filled  with  the  liquid.  It  is  now  softened 
in  the  blowpipe  flame  at  a  few  centimetres  from  the  open 
end,  and  the  fused  glass  allowed  to  thicken,  and  it  is  then 
drawn  out  into  a  thick-walled  capillary  tube.  The  tube  is 
supported  in  the  clamp  of  a  retort  stand,  and  the  nitric  acid 
caused  to  boil,  so  as  to  expel  the  air  contained  within  the 
tube  :  when  the  acid  vapours  are  freely  evolved,  the  lamp  is 
removed,  and  the  capillary  opening  is  closed  by  the  blowpipe 
flame.  Allow  the  liquid  to  become  nearly  cold,  wrap  the 
tube  in  a  thick  towel  (for  safety),  and  break  the  bulb  by 
shaking  it  smartly  against  the  ends  of  the  tube.  Heat  the 
tube  to  120-150°  for  some  hours  in  the  air-bath.  Allow  the 
bath  to  cool  before  withdrawing  the  tube,  wrap  it  in  the 
towel,  and  cautiously  warm  the  point,  so  as  to  expel  the  liquid 
which  collects  in  the  capillary  tube.  Soften  the  end  in  the 
blowpipe  flame  :  the  enclosed  gases  will  force  their  way 
through  the  fused  glass.  Examine  the  tube  carefully,  and  if 
you  have  reason  to  believe  that  the  oxidation  is  incomplete, 
re-seal  the  tube  and  heat  it  to  180°  for  an  hour.  Allow  it  to 
cool  and  open  it  with  the  same  precautions  as  before.  If  no 
more  gas  escapes  the  process  is  finished.  Cut  off  the  end  of 


368  Quantitative  Chemical  A  nalysis. 

the  tube,  rinse  its  contents  into  a  beaker,  dilute  with  water, 
and,  in  the  case  of  sulphur,  add  barium  chloride.  In  the 
case  of  phosphorus,  add  ammonia,  ammonium  chloride,  and 
magnesia- mixture,  and  convert  the  precipitate  into  magnesium 
pyrophosphate.  If  sulphur  and  phosphorus  are  together 
present,  precipitate  the  sulphuric  acid  with  barium  chloride, 
remove  the  excess  of  baryta  by  sulphuric  acid,  concentrate 
the  filtrate  by  evaporation,  and  determine  the  phosphoric 
acid  as  magnesium  pyrophosphate. 


APPENDIX. 


TABLE  I. 
Symbols  and  Atomic  Weights  of  the  Elements. 


Element 

Symbol 

Atomic  weight 

Observer 

Aluminium 

Al 

27-02 

Mallet 

Antimony 

Sb 

II9-6 

Schneider  ;  Cooke 

Arsenic    . 

As 

74*9 

Kessler 

Barium     . 

Ba 

136-84 

Marignac 

Bismuth  . 

Bi 

207-5 

Dumas 

Boron 

B 

10-9 

Berzelius 

Bromine  . 

Br 

79-76 

Stas 

Cadmium 

Cd 

1117 

Lenssen 

C  cesium    . 

Cs 

132-7 

Johnson  and  Allen  ;  Bunsen 

Calcium   . 

Ca 

39-90 

Erdmann  and  Marchand 

Carbon     . 

C 

11-97 

Dumas  and  Stas  ;  Liebig 

Cerium     . 

Ce 

138-24 

Rammelsberg 

Chlorine  . 

Cl 

35'37 

Stas 

Chromium 

Cr 

52-08 

Siewert 

Cobalt      . 

Co 

58-6 

Russell 

Copper     . 

Cu 

63-12 

Millon  and  Commaille 

Didymium 

D 

142-44 

Hermann 

Erbium    . 

E 

168-9 

Bahr  and  Bunsen 

Fluorine  . 

F 

18-96 

Luca  ;  Louyet 

Gallium    . 

Ga 

69-8 

Lecoq  de  Boisbaudran 

Glucinum 

Gl 

9-30 

Awdejew  ;  Klatzo 

Gold 

Au 

196-85 

Thorpe  and  Laurie 

Hydrogen 

H 

i 

Dulong  and  Berzelius 

Indium     . 

In 

H3'4 

Winkler;  Bunsen 

Iodine 

I 

126-54 

Stas 

Indium    . 

Ir 

192-5 

Seubert 

f 

B  "3 


370 


Appendix. 
TABLE  I. — continued. 


Element 

Symbol 

Atomic  weight 

Observer 

Iron 

Fe 

55'9 

Dumas 

Lanthanum 

La 

I39-33 

Hermann 

Lead        . 

Pb 

2O6  '40 

Stas 

Lithium    .      .   . 

Li 

7'00 

Stas 

Magnesium 

Mg 

23-94 

Dumas 

Manganese 

Mn 

54'8 

Dewar  and  Scott 

Mercury  . 
Molybdenum    . 
Nickel      .         ." 

Hg 
Mo 

Ni 

199-8 

95-9 
58-6 

Erdmann  and  Marchand 
Dumas  ;  Debray 
Russell 

Niobium  . 

Nb 

93'7 

Marignac 

Nitrogen  . 

N 

14-01 

Stas 

Osmium  .    '     . 

Os 

190-8 

Seubert 

Oxygen    .         . 

0 

15-96 

Nilson 

Palladium         J 

Pd 

106-2 

Berzelius 

Phosphorus 

P 

30-96 

Schrotter 

Platinum.      '  . 

Pt 

194-38 

Seubert 

Potassium       v  . 

K 

39-04 

Stas 

Rhodium 

Rh 

104-1 

Berzelius 

Rubidium     v    . 

Rb 

85-2 

Bunsen  ;  Piccard 

Ruthenium 

Ru 

I03-5 

Berzelius 

Scandium 

Sc 

44-0 

Nilson 

Selenium  . 

Se 

78-9 

Dumas 

Silver 

Ag 

107-67 

Stas 

Silicon     . 

Si  ' 

28-33 

Thorpe  and  Young 

Sodium    . 

Na 

22-99 

Stas 

Strontium 

Sr 

8734 

Marignac 

Sulphur   . 

S 

31-996 

Stas 

Tantalum 

Ta 

182-00 

Marignac 

Tellurium     .     . 

Te 

125-0 

Brauner 

Thallium 

Tl 

203-50 

Crookes 

Thorium  . 

Th 

231-44 

Delafontaine 

Tin.  .      . 

Sn 

117-4 

Dumas 

Titanium 

Ti 

48-0 

Thorpe 

Tungsten 

W 

183-6 

Schneider  ;  Dumas;  Roscoe 

Uranium  . 

U 

239-8 

Ebelmen 

Vanadium 

V 

51-0 

Roscoe 

Ytterbium 

Yt 

173-0 

Nilson 

Yttrium    . 

Y 

88-9 

Bahr  and  Bunsen 

Zinc 

Zn 

647 

Axel  Erdmann 

Zirconium 

Zr 

90-4 

Marignac  ;  Bailey 

Appendix. 


37* 


TABLE  II. 
Volume  and  Density  of  Water  at  different  Temperatures. 

(Mean  results  of  the  observations  of  Kopp,  Pierre,   Despretz,  Hagen, 
Matthiessen,  Weidner,  Kremers,  and  Rossetti.) 


Temp. 

Sp.  gr.  of  Water 

(at  o°  =  i) 

Vol.  of  Water 
(at  o°  =  i) 

Sp.  gr.  of  Water 
(at  4°  =  i) 

Volume  of  Water 
(at4°=i) 

O 

I  -000000 

I  -OOOOOO 

•999871 

•000129 

I 

i  -00005  7 

o  '999943 

•999928 

•000072 

2 

1-000098 

•999902 

•999969. 

•OOOO3I 

3 

I  -0001  20 

•999880 

•999991 

•OOOOO9 

4 

1-000129 

•999871 

I  -000000 

•oooooo 

5 

1-000119 

•99988l 

0-999990 

•ooooio 

6 

I  -000099 

•999901 

•999970 

•000030 

7 

I  -000062 

•999938 

'999933 

•000067 

8 

1-000015 

•999985 

•999886 

1-000114 

9 

0-999953 

I  -000047 

•999824 

1-000176 

10 

•999876 

I-OOOI24 

'999747 

I  -000253 

ii 

•999784 

I  -OOO2I6 

•999655 

I  -000345 

12 

•999678 

I  -000322 

'999549 

1-000451 

13 

'999559 

I-OOO44I 

•999430 

1-000570 

H 

•999429 

I-000572 

•999299 

I  -000701 

15 

•999289 

I-0007I2 

•999160 

I  -000841 

16 

•999131 

I  -000870 

•999002 

I  -000999 

17 

•998970 

I-OOIO3I 

•998841 

I  -001  1  60 

18 

•998782 

I-OOI2I9 

•998654 

1-001348 

19 

•998588 

1-001413 

•998460 

1-001542 

20 

•998388 

I-OOI6I5 

•998259 

1-001744 

21 

•998176 

I  -001828 

•998047 

1-001957 

22 

'997953 

I  -002049 

•997826 

1-002177 

23 

•997730 

I  -002276 

•997601 

I  -002405 

24 

'997495 

I  -0025  1  1 

•997367 

1-002641 

25 

•997249 

I  -002759 

•997120 

I  -002888 

26 

•996994 

I  -003014 

•996866 

I  -003144 

27 

•996732 

I  -003278 

•996603 

I  -003408 

28 

•996460 

1  -003553 

•996331 

I  -003682 

29 

•996179 

1-003835 

•996051 

I  -003965 

30 

•995894 

I-004I23 

•995765 

I  -004253 

35 

•99431 

I-00572 

•99418 

I  -00586 

40 

•99248 

1-00757 

•99235 

I  -00770 

5o 

•98833 

I  -OIlSl 

•98820 

1-01195 

60 

•98351 

I-OI677 

•98338 

1-01691 

70 

•97807 

I  -O2243 

'97794 

I  -02256 

80 

•97206 

I  -02874 

•97194 

I  -02887 

90 

•96568 

1-03554 

•96556 

I  -03567 

100 

•95878 

I  -04300 

•95865 

1-04312 

B  B  2 


372 


Appendix. 


TABLE 
Tension  of  Aqueous  Vapour  in 


i 

0 

Mm. 

o 

Mm. 

o 

Mm. 

o 

Mm. 

o 

Mm. 

0 

Mm. 

0 

Mm. 

o'o 

4-600 

2-5 

5  '49i 

S'o 

6  '534 

7  '5 

7'75i 

10  '0 

9'i6S 

12-5 

10  '804 

15  '0 

12-699 

'I 

•633 

•6 

'530 

'i 

•580 

•6 

•804 

'i 

'227 

•6 

•875 

•i 

•781 

'2 

•667 

'7 

i 

*2 

•625 

'7 

'857 

*2 

•288 

'7 

'947 

'2 

•864 

'3 

•700 

•8 

•608 

'3 

•671 

•8 

•910 

'3 

'350 

•8 

1  1  '019 

'3 

'947 

'4 

'733 

'9 

•647 

'4 

•717 

'9 

•964 

'4 

•412 

'9 

•090 

•4 

13-029 

"5 

•767 

3'° 

•687 

'5 

'763 

8'o 

8-017 

'5 

'474 

13  '0 

•162 

•5 

"112 

•6 

•801 

'i 

'727 

•6 

'810 

'i 

•072 

•6 

'537 

•i 

'235 

•6 

-197 

'7 

•836 

*2 

•767 

'7 

•857 

'2 

•126 

'7 

•601 

"2 

'309 

'7 

•28l 

•8 

•871 

'3 

•807 

*8 

•904 

'3 

•181 

'8 

•665 

'3 

'383 

•8 

•366 

'9 

'90S 

'4 

•848 

'9 

'951 

'4 

•236 

'9 

"728 

'4 

•456 

'9 

•451 

I'O 

•940 

'5 

•889 

6'o 

•998 

"5 

•291 

I  I'O 

'792 

'5 

'530 

i6"o 

•536 

'i 

'975 

•6 

'930 

•i 

7-047 

•6 

'347 

'I 

•857 

•6 

'605 

'i 

•623 

'2 

S'on 

'7 

'972 

'2 

'095 

'7 

•404 

'2 

'923 

'7 

•68  1 

'2 

'710 

'3 

'047 

•8 

6*014 

'3 

'144 

•8 

•461 

'3 

'989 

'8 

'757 

'3 

'797 

4 

'5 

'082 
•118 

y 
4-0 

'055 
•097 

'4 

'5 

'242 

9 
9'o 

'574 

4 
'5 

10-054 

'120 

y 
14-0 

•832 
•908 

'4 

*5 

•885 
'972 

'6 

•i55 

•I 

•140 

*6 

•292 

'I 

'632 

•6 

•l87 

'I 

•986 

•6 

14*062 

'7 

'191 

'2 

•183 

*7 

'342 

"2 

"690 

'7 

•255 

'2 

12-064 

"7 

•151 

•8 

•228 

•3 

•226 

*8 

'392 

'3 

•748 

•8 

•322 

•3 

•142 

•8 

-24I 

'9 

'265 

'4 

•270 

'9 

•442 

"4 

•807 

'9 

•389 

'4 

'220 

'9 

'331 

2'0 

'302 

'5 

'3i3 

7'o 

'492 

'5 

•865 

I2'0 

'457 

"5 

•298 

17-0 

'421 

'I 

"340 

•6 

'357 

'I 

'544 

•6 

•925 

'I 

•526 

•6 

•378 

'i 

'5*3 

'2 

•378 

'7 

•407 

*2 

'595 

'7 

'985 

'2 

'596 

'7 

•458 

'2 

•605 

'3 

•416 

•8 

'445 

3 

'647 

•8 

9  '°45 

'3 

'665 

'8 

•538 

'3 

•697 

'4 

'454 

'9 

•490 

'4 

•699 

'9 

•105 

"4 

'734 

'9 

'619 

'4 

•790 

Appendix. 


373 


III. 

Millimetres  of  Mercury  from  o°  to  34*9°  C. 


0 

Mm. 

0 

Mm. 

o 

Mm. 

0 

Mm. 

o 

Mm. 

0 

Mm.. 

° 

Mm. 

i?  '5 

14-882 

20  '0 

17  '391 

22-5 

20-265 

25  'o 

23  '55o 

27  '5 

27-294 

30  "o 

3i'548 

32  '5 

36  '37° 

•6 

'977 

'I 

•500 
'608 

•6 

•389 

'i 

•692 
"874. 

•6 

'455 

'I 

'729 

•6 

•576 
•7g.j 

'7 
•8 

[5    OJS 

•167 

'3 

•717 

7 

•8 

5*4 
'639 

'3 

°34 
•976 

7 
•8 

•778 

•3 

32*094 

•8 

7°3 
•991 

'9 

•262 

'4 

•826 

'9 

•763 

'4 

24-119 

'9 

'939 

'4 

•278 

'9 

37-200 

i8'o 

'357 

'5 

'935 

23  'o 

•888 

'5 

•261 

28-0 

28'IOI 

'5 

'463 

33  'o 

•410 

'i 

'454 

•6 

18-047 

'i 

21  'Ol6 

•6 

•406 

'i 

•267 

•6 

'650 

"i 

•621 

"2 

'552 

"7 

•i59 

-2 

•144 

'7 

'552 

"2 

'433 

'7 

'837 

'2 

'832 

'3 

'650 

•8 

•271 

'3 

•272 

•8 

•697 

'3 

'599 

•8 

33-026 

'3 

38  '045 

'4 

'747 

'9 

•383 

'4 

•400 

'9 

'842 

'4 

'765 

'9 

'215 

'4 

'258 

'5 

•845 

21  '0 

'495 

'5 

•528 

26*0 

•988 

'5 

'93  1 

31  'o 

'405 

'5 

'473 

•6 

'7 

'945 

'I 

*2 

"610 

•6 

•659 

'i 

25'i38 

•6 

2g'ioi 

'* 

•596 
•787 

•6 
•7 

•689 
'006 

/ 

•8 

10  045 

•145 

'3 

•839 

•8 

•921 

"3 

'438 

•8 

•441 

'3 

y0/ 
•980 

7 

•8 

yoo 
39'i24 

'9 

'246 

'4 

'954 

'9 

22-058 

'4 

•588 

'9 

•612 

'4 

34'i74 

'9 

'344 

19-0 

•346 

'5 

19-069 

24-0 

•184 

'5 

•738 

29-0 

•782 

'5 

'368 

34  'o 

•565 

•i 

•449 

•6 

•187 

'i 

•319 

•6 

•891 

'i 

•956 

•6 

'564 

"i 

•786 

'2 

•552 

'7 

'305 

'2 

'453 

'7 

26-045 

'2 

30-131 

"7 

•76r 

'2 

40-007 

'3 

•655 

•8 

'423 

'3 

•588 

•8 

•198 

'3 

'305 

•8 

'959 

'3 

'230 

'4 

•758 

*9 

'S4i 

'4 

'723 

'9 

'351 

'4 

'479 

'9 

35'i59 

'4 

'455 

'5 

•861 

22  '0 

'659 

'5 

•858 

27-0 

'SOS 

'5 

•654 

32  'o 

'359 

'5 

•680 

•6 

•967 

'I 

•780 

•6 

•996 

'i 

•663 

•6 

•833 

'i 

'559 

•6 

•907 

'7 

!7'073 

*2 

•901 

'7 

23<:I35 

'2 

'820 

'7 

31  'on 

'2 

•760 

'7 

4i'i35 

•8 

•179 

'3 

2O  "O2  2 

•8 

'273 

'3 

•978 

•8 

•190 

'3 

•962 

•8 

'364 

'9 

•285 

'4 

'r43 

'9 

•411 

'4 

27-136 

'9 

•369 

'4 

1 

36-165 

'9 

'595 

374 


Appendix. 


TABLE  IV. 

BAUME'S   HYDROMETER. 
Table  for  Liquids  heavier  than  Water. 


Degrees 
Baume 

Sp.gr. 

OB. 

Sp.  gr. 

•R 

Sp.  gr. 

0 

I  -000 

26 

•206 

52 

1-520 

I 

1-007 

27 

•216 

53 

i  -535 

2 

I-OI3 

2§ 

•226 

54 

1-551 

3 

1-020 

29 

-236 

55 

1-567 

4 

1-027 

30 

•246 

56 

5 

1-034 

31 

•256 

57 

i  -600 

6 

I-04I 

32 

•267 

58 

1-617 

7 

1-048 

33 

•277 

59 

1-634 

8 

1-056 

34 

•288 

60 

1-652 

9 

1-063 

35 

•299 

61 

1-670 

10 

I-O7O 

36 

•310 

62 

1-689 

ii 

I-078 

37 

•322 

63 

1-708 

12 

I  -086 

38 

•333 

64 

1-727 

13 

•094 

39 

'345 

65 

1-747 

14 

•101 

40 

'357 

66 

1-767 

15 

•109 

-369 

67 

1-788 

16 

•118 

42 

•382 

68 

1-809 

17 

•126 

43 

•395 

69 

1-831 

18 

•134 

44 

•407 

70 

i  -854 

19 

•143 

45 

•421 

1-877 

20 

•152 

46 

'434 

72 

1-900 

21 

•160 

47 

•448 

73 

1-924 

22 

•169 

48 

•462 

74 

1-949 

23 

•178 

49 

•476 

75 

1-974 

24 

•188 

50 

•490 

76 

2-OOO 

25 

.   '197 

51 

•505 

Appendix. 


375 


TABLE  IV. — continued. 
Table  for  Liquids  lighter  than  Water. 


°B. 

Sp.  gr. 

°B. 

Sp.  gr. 

B. 

Sp.  gr. 

IO 

I  -000 

27 

0-896 

44 

0-811 

II 

0-993 

28 

0-890 

45 

0-807 

12 

0-986 

29 

0-885 

46 

0-802 

13 

0-980 

3° 

0-880 

47 

0-798 

H 

0-973 

3i 

0-874 

48 

0-794 

15 

0-967 

32 

0-869 

49 

0-789 

16 

0-960 

33 

0-864 

50 

0-785 

17 

o-954 

34 

0-859 

5i 

0-781 

18 

0-948 

35 

0-854 

52 

0-777 

19 

0-942 

36 

0-849 

53 

0-773 

20 

0-936 

37 

0-844 

54 

0-768 

21 

0-930 

38 

0-839 

55 

0-764 

22 

0-924 

39 

0-834 

56 

0-760 

23 

0-918 

40 

0-830 

57 

0757 

24 

0-913 

4i 

0-825 

58 

o-753 

25 

0-907 

42 

0-820 

59 

0-749 

26 

0-901 

43 

0-816 

60 

0745 

TWADDELL'S   HYDROMETER. 

To  convert  degrees  Twaddell  into  specific  gravity  (water 
multiply  the  number  by  5,  and  add  1,000  to  the  product. 


i.ooo)  : 


To  reduce  specific  gravity  (water  =  1,000)  to  Twaddell  :    deduct 
1,000  and  divide  the  remainder  by  5. 


376 


Appendix. 


TABLE  V. 

Showing  the  Percentages  of  real  Sulphuric  Acid 

corresponding  to  various  Specific  Gravities  of  Aqueoits  Sul 
phuric  Acid. 

^ineau  j  Otto.     Temp.  15°. 


Specific 
gravity 

Per 

cent. 

Specific 
gravity 

Per 

cent. 

Specific 
gravity 

Per 

cent. 

Specific 
gravity 

Per 

cent. 

I  "8426 

100 

1-675 

75 

•398 

50 

I-182 

25 

1-842 

99 

1-663 

74 

•3886 

49 

I-I74 

24 

I  '8406 

98 

I-65I 

73 

•379 

48 

•I67 

23 

1-840 

97 

1-639 

72 

•370 

47 

•159 

22 

1-8384 

96 

1-627 

7i 

•361 

46 

•1516 

21 

1-8376 

95 

1-615 

70 

•35i 

45 

•144 

2O 

I-83S6 

94 

1-604 

69 

•342 

44 

•I36 

19 

1-834 

93 

I-592 

68 

'333 

43 

•129 

18 

1-831 

92 

1-580 

67 

•324 

42 

•121 

17 

1-827 

9i 

I-568 

66 

•315 

41 

•1136 

16 

1-822 

90 

i'557 

65 

•306 

40 

•106 

15 

1-816 

89    . 

1-545 

64 

•2976 

39 

•098 

14 

1-809 

88 

1-534 

63 

•289 

38 

•091 

13 

1-802 

87 

1-523 

62 

•281 

37 

1-083 

12 

i  -794 

86 

1-512 

61 

•272 

36 

1-0756 

II 

i  -786 

85 

1-501 

60 

•264 

35 

1-068 

10 

1-777 

84 

1-490 

59 

•256 

34 

1-061 

9 

1-767 

83 

1-480 

58 

•2476 

33 

i  -0536 

8 

1-756 

82 

1-469 

57 

•239 

32 

i  -0464 

7 

1-745 

81 

1-4586 

56 

•231 

3i 

1-039 

6 

1-734 

80 

1-448 

55 

•223 

30 

1-032 

5 

1-722 

79 

I-438 

54 

•215 

29 

i  -0256 

4 

1-710 

78 

1-428 

53 

•2066 

28 

1-019 

3 

1-698 

77 

1-418 

52 

•198 

27 

1-013 

2 

1-686 

76 

1-408 

5i 

•190 

26 

1-0064 

I 

Appendix. 


377 


TABLE  VI. 

Giving  the  Percentage  Amount  of  Hydrochloric  Add  contained 
in  Aqueous  Solutions  of  the  Gas  of  various  Specific  Gravities. 

Ure.     Temp.  15°. 


Specific 

HC1 

Specific 

HC1 

Specific 

HC1 

Specific 

HC1 

gravity 

per  cent. 

gravity 

percent. 

gravity 

per  cent. 

gravity 

per  cent. 

•2OOO 

40777 

•1515 

30-582 

I  -1000 

20-388 

I  -0497 

10-194 

•1982 

40-369 

•1494 

30-174 

I  -0980 

19-980 

I  -0477 

9-786 

•1964 

39-961 

•1473 

29-767 

I  -0960 

19-572 

1-0457 

9379 

•1946 

39-554 

•1452 

29359 

I  -0939 

19-165 

I  -0437 

8-971 

•1928 

39-146 

•1431 

28-951 

I  -0919 

18-757 

I-04I7 

8-563 

T9IO 

38-738 

•1410 

28-544 

I  -0899 

I8-349 

I  -0397 

8-155 

•I893 

38-330 

•1389 

28-136 

I  -0879 

17-941 

I  -0377 

7-747 

•1875 

37-923 

•1369 

27-728 

I  -0859 

I7-534 

1-0357 

7340 

•1857 

37-5I6 

•1349 

2732I 

I  -0838 

17-126 

I  -0337 

6-932 

•1846 

37-108 

•1328 

26-913 

I  -0818 

16-718 

I-03I8 

6-524 

•1822 

36-700 

•1308 

26-505 

I  -0798 

I6-3IO 

I  -0298 

6-II6 

•1802 

36-292 

•1287 

26-098 

1-0778 

15-902 

I  -0279 

5709 

•1782 

35-884 

•1267 

25-690 

I  -0758 

I5-494 

I  -0259 

5-301 

•1762 

35-476 

•1247 

25-282 

I  -0738 

15-087 

I  -0239 

4^93 

•1741 

35-068 

•1226 

24^74 

1-0718 

14-679 

I  -0220 

4-486 

•1721 

34-660 

•I2O6 

24-466 

•0697 

14-271 

1-0200 

4-078 

•I7OI 

34-252 

•Il85 

24-058 

•0677 

13-863 

I'OlSo 

3^70 

•1681 

33-845 

•1164 

23-650 

•0657 

13-456 

I  'Ol6o 

3-262 

•1661 

33-437 

•1143 

23-242 

•0637 

13-049 

I  -0140 

2-854 

•1641 

33-029 

•U23 

22-834 

•0617 

12-641 

I  -01  20 

2-447 

•1620 

32-62I 

•IIO2 

22  -426 

I  -0597 

12-233 

I'OIOO 

2-039 

•1599 

32-2I3 

•1082 

22-OI9 

1-0577 

II-825 

I'OOSO 

I  '631 

1-1578 

31-805 

•1061 

2I-6II 

1-0557 

11-418 

I  -OO6O 

IT24 

I-I557 

3I-398 

•1041 

21-203 

1-0537 

II'OIO 

I-0040 

0-816 

IT536 

30-990 

•IO2O 

20-796 

1-0517 

10-602 

I  -0020 

0-408 

373 


Appendix. 
TABLE  VII. 


Showing  the  Percentage  Amount  of  Nitric  Acid  (HNO3) 
contained  in  Aqueous  Solutions  of  various  Specific  Gravities. 

(Kolb,  Ann.  Ch.  Phys.  [4]  136). 

The  numbers  marked  *  are  the  results  of  direct  observations  ;  the 
others  are  obtained  by  interpolation. 


HNO3 

per  cent. 

Specific  gravity 

Contrac- 
tion 

HNO3 

per  cent. 

Specific  gravity 

Contrac- 
tion 

Ato° 

At  15° 

Ato° 

At  15° 

lOO'OO 

J'559 

I-530 

O'OOOO 

68-00 

1-435 

1-414 

0-0784 

99-84* 

1-559* 

1-530* 

O-OOO4 

67-00 

I-430 

I  -410 

0-0796 

9972* 

1-558* 

1-530* 

O'OOIO 

66-00 

I-425 

I-405 

0-0806 

99-52* 

1*557* 

1-529* 

O-OOI4 

65-07* 

I  -420* 

1-400* 

0-0818 

97-89* 

i-55i* 

1-523* 

0-0065 

64-00 

I'4I5 

1-395 

0-0830 

97-OO 

I-548 

1-520 

0-0090 

63-59 

I-4I3 

1-393 

0-0833 

96-00 

1-544 

1-516 

0-0120 

62-00 

1-404 

1-386 

0-0846 

95-27* 

1-542* 

1-514* 

0-OI42 

61-21* 

I  -400* 

1-381* 

0-0850 

94-00 

1-537 

1-509 

0-OI82 

60-00 

1-393 

1-374    i  0-0854 

93-01* 

i-533* 

I  -506* 

0-0208 

59-59* 

1-391* 

1-372* 

0-0855 

92-00 

1-529 

I-503 

0-0242 

58-88 

1-387 

1-368 

0-086  1 

91-00 

1-526 

1-499 

O-O272 

58-00 

1-382 

1-363 

0-0864 

90-00 

1-522 

1-495 

O-O3OI 

57-oo 

1-376 

I-358 

0-0868 

89-56* 

1-521* 

I  -494* 

0-0315 

56-10* 

i-37i* 

1-353* 

0-0870 

88-00 

i-5i4 

1-488 

0-0354 

55-oo 

I-365 

1-346 

0-0874 

87-45* 

1-513* 

I  -486* 

0-0369 

54-oo 

1-359 

i-34i 

0-0875 

86-17* 

1-507* 

1-482 

0-0404 

53-8i 

I-358 

1-339 

0-0875 

85-00 

1-503 

1-478 

0-0433 

53-oo 

1-353 

1-335 

0-0875 

84-00 

1-499 

1-474 

0-0459 

52-33* 

i  -349* 

i-33i* 

0-0875 

83-00 

1-495 

1-470 

0-0485 

50-99* 

1-341* 

1-323*  i  0-0872 

82-00 

1-492 

1-467 

0-0508 

49-97 

1-334 

1-317    j  0-0867 

80-96* 

1-488* 

I-463* 

0-053I 

49-00 

1-328 

1-312 

0-0862 

80-00 

1-484 

1-460 

0-0556 

48-00 

1-321 

1-304 

0-0856 

79-00 

1-481 

I-456 

0*0580 

47-18* 

i-3i5* 

1-298* 

0-0850 

77'66 

1-476 

I-451 

0-0610 

46-64 

1-312 

1-295 

0-0848 

76-00 

1-469 

1-445 

0-0643 

45-00 

1-300 

1-284 

0-0835 

75-00 

1-465 

1-442 

0-0666 

43-53* 

1-291* 

i  -274* 

0-0820 

74-01* 

i  -462* 

1-438* 

0-0688 

42-00 

1-280 

1-264 

0-0808 

73-00 

1-457 

1-435 

0-0708 

41-00 

1-274 

1-257 

0-0796 

72-39* 

1-455* 

1-432* 

0-0722 

40-00 

1-267 

1-251 

0-0786 

71-24* 

i  -450* 

i  -429* 

0-0740 

39-00 

1-260 

1-244 

0-0755 

69-96 

1-444 

1-423 

0-0760 

37-95* 

1-253* 

1-237* 

0-0762 

69-20* 

1-441 

1-419* 

0-0771 

36-00 

1-240 

1-225 

0-0740 

Appendix. 
TABLE  VII.—  continued. 


379 


Specific  gravity 

Specific  gravity 

HNO3 

Contrac- 

HNO3 

Contrac- 

tion 

tion 

per  cent. 

Ato° 

At  15° 

per  cent. 

Ato° 

At  15° 

35'°° 

•234 

I-2I8 

0-0729 

2O-OO 

I-I32 

I  -120 

0*0483 

33-86* 

•226* 

I-2II* 

0-0718 

17-47* 

1-115* 

I-I05* 

0-0422 

32-00 

•214 

•198 

0-0692 

15-00 

1-099 

1-089 

0-0336 

31-00 

•207 

•192 

0-0678 

I3-00 

1-085 

1-077 

0-0316 

30-00 

•200 

•I85 

0-0664 

II-4I* 

1-075 

I  -067* 

0*0296 

29-00 

•194 

•179 

0-0650 

7-22* 

1-050 

I  -045* 

0-0206 

28-00* 

•I87* 

•I72* 

0-0635 

4-OO 

1-026 

1-022 

0-OII2 

27-00 

•180 

•166 

0-0616 

2-00 

1-013 

I  -010 

0-0055 

2571* 

1-171* 

•157* 

0-0593 

o-oo 

I  -000 

0-999 

o-oooo 

23-00 

I-I53 

•138 

0-0520 

TABLE  VIII. 

Showing  the  Percentage  Amount  of  Caustic  Potash 

Aqueous  Solutions  of  various  Specific  Gravities. 

Tiinnermann,  N.  Tr.  xviii.,  2,  5.  Temp.  15°. 


n 


Sp.  gr. 

Per  cent. 

Sp.  gr. 

Per  cent. 

•3300 

28-290 

•1437 

I4-I45 

•3131 

27-158 

•1308 

13-013 

•2966 

26-027 

•Il82 

1  1  -882 

•2805 

24-895 

•1059 

IO-750 

•2648 

23764 

•0938 

9-619 

•2493 

22-632 

•0819 

8-487 

•2342 

21-500 

•0703 

7-355 

•2268 

20-935 

I  -0589 

6-224 

•2122 

19-803 

I  -0478 

5-002 

•1979 

18-671 

I  -0369 

3-961 

•l839 

I7-540 

I  -0260 

2-829 

•1702 

16-408 

I-OI53 

1-697 

•1568 

15-277 

I  -0050 

0-5658 

3^0 


Appendix. 


TABLE  IX. 

Showing  Percentage  Amount  of  Soda  (Na2O)  in  Aqueous 

Solutions  of  various  Specific  Gravities. 

Tiinnermann. 


Sp.  gr. 

Per  cent. 

Sp.gr. 

Per  cent. 

Sp.gr. 

Per  cent. 

Sp.  gr. 

Per  cent. 

•4285 

30-220 

•3198 

22-363 

1-2392 

I5'IIO 

1-1042 

7-253 

•4193 

29-616 

•3H3 

21-894 

•2280 

14-500 

I  -0948 

6-648 

•4101 

29-011 

•3125 

21-758 

•2178 

13-901 

I-0855 

6-044 

•4011 

28-407 

•3053 

21-154 

-2058 

13-297 

I  -0764 

5-440 

•3923 

27-802 

•2982 

20-550 

•1948 

12-692 

1-0675 

4-835 

•3836 

27-200 

•2912 

I9-945 

•1841 

12-088 

I  -0587 

4-231 

•3751 

26-594 

•2843 

I9-34I 

•1734 

1  1  -484 

I  -0500 

3-626 

•3668 

25-989 

•2775 

18-730 

•1630 

10-879 

1-0414 

3-022 

•3586 

25385 

•2708 

18-132 

•1528 

10-275 

I  -0330 

2-418 

•3505 

24-780 

•2642 

I7-528 

•1428 

9-670 

I  -0246 

I-8I3 

•3426 

24-176 

•2578 

16-923 

•1330 

9-066 

1-0163 

1-209 

'3349 

23-572 

•2515 

16-379 

•1233 

8-462 

I  -008  1 

0-604 

•3273 

22-967 

•2453 

I5-7I4 

•1137 

7-857 

1-0040 

0-302 

TABLE  X. 

Showing  the  Percentage  Amount  of  Ammonia  in  Aqueous 

Solutions  of  the  Gas  of  various  Specific  Gravities. 

Carius.  Temp.  14°. 


Sp.  gravity 

NH3 

per  cent. 

Sp.  gravity 

NH3 

percent. 

Sp.  gravity 

NHS 
percent. 

0-8844 

36 

0-9133 

24 

0-9520 

12 

0-8864 

35 

0-9162 

23 

0-9556 

II 

0-8885 

34 

0-9191 

22 

0-9593 

10 

0-8907 

33 

0-9221 

21 

0-9631 

9 

0-8929 

32 

0-9251 

2O 

0-9670 

8 

0-8953 

3i 

0-9283 

'9 

0-9709 

7 

0-8976 

30 

0-9314 

18 

0-9749 

6 

0-9001 

29 

0-9347 

17 

0-9790 

5 

0-9026 

28 

0-9380 

16 

0-9031 

4 

0-9052 

27 

0-9414 

15 

0-9873 

3 

0-9078 

26 

0-9449 

'4 

0-9915 

2 

0-9106 

25 

0-9484 

13 

0'9959 

I 

Appendix. 


TABLE  XI. 

Reduction  of  Weighings  in  Air  to  a  Vacuum  (G.  F.  Becker •, 
Liebig's  Annalen,  1 9  5 ,  p.  2  2  2 ). 


Brass  weights 
for  substances  whose  sp.  gr. 
is  between 

Correction  per 
gram,  error  less 
than  3*0  mgrm. 

Platinum  weights 
for  substances  whose  sp.  gr. 
is  between 

27738  and  11-064 

—  O-OOOO67 

51-766  and  13-568 

1  1  -064 

6-904 

O'OOOOOO 

13-568 

7-807 

6-904 

5-019 

+  0-000067 

7-807 

5-480 

5-019 

3-943 

0-000133 

5-480 

4*222 

3-943 

3-247 

0-000200 

4-222 

3'433 

3-247 

2759 

O-OOO267 

3-433 

2-893 

2759 

2-399 

O-OOO333 

2-893 

2-500 

2-399 

2*122 

0-000400 

2-500 

2-201 

2'122 

I-903 

0-000467 

2-201 

•965 

•903 

1724 

0-000533 

•965 

•776 

724 

1-576                     O-OOO6OO 

•776 

•619 

•576 

I-452 

O-OOO667 

•619 

•488 

•452 

1-377 

0-000733 

•488 

•377 

'377 

1-254 

0-000800 

'377 

•281 

•254 

I-J74    . 

0-000867 

•28l 

•197 

•174 

1-103 

O-OOO933 

•I97 

•124 

•103 

1-041 

o-ooiooo 

•124 

•059 

1-041 

0-985 

0-001067 

•059 

1-002 

0-001133 

•002 

0-950 

O-OOI2OO 

PREPARATION    OF    PURE    PLATINUM    TETRA- 
CHLORIDE. 

Scrap  platinum,  which  may  contain  iridium,  osmium,  &c.,  is  dissolved 
in  aqua  regia,  and  the  solution  is  evaporated  to  dryness ;  the  residue  is 
dissolved  in  moderately  concentrated  hydrochloric  acid,  and  again 
evaporated  to  dryness.  The  dried  chloride  is  once  more  dissolved  in 
hot  water  containing  free  hydrochloric  acid,  and  the  solution  mixed 
with  a  large  excess  of  soda-ley.  It  is  again  boiled  for  some  time,  and  a 
few  drops  of  alcohol  are  added  in  order  to  destroy  any  sodium  hypo- 
chlorite  which  may  be  formed  ;  the  precipitate  is  redissolved  in  hydro- 
chloric acid ;  the  liquid  is  filtered,  if  necessary,  and  mixed  with  a  hot 
ana  saturated  solution  of  ammonium  chloride  so  long  as  a  precipitate 
forms.  This  process  of  separating  platinum  from  its  congeners,  with 
which  in  the  commercial  variety  of  the  metal  it  is  almost  invariably 


382  Appendix. 

mixed,  is  based  upon  the  different  behaviour  of  sodium  hydrate  solution 
towards  the  higher  chlorides  of  the  associated  metals.  Platinic  chloride 
is  very  slightly,  if  at  all,  reduced  to  platinous  chloride  on  boiling  with 
soda  solution,  whereas  the  other  chlorides  are  all  reduced,  with  the  pro- 
duction of  sodium  chloride  and  hypochlorite,  and  these  reduced  chlorides 
are  not  precipitated  in  union  with  ammonium  chloride. 

The  ammonium-platinum  chloride,  which  is  of  a  bright  yellow  colour 
and  free  from  orange  or  red  crystals,  is  washed  by  decantation,  dried, 
and  gently  heated  in  a  platinum  crucible,  or  it  may  be  placed  in  a  piece 
of  hard  glass  tubing  and  decomposed  in  a  current  of  coal  gas  or  dry 
hydrogen.  The  reduced  metal  should  be  weighed,  dissolved  in  aqua 
regia,  the  solution  evaporated  to  dryness  with  excess  of  hydrochloric  acid 
to  expel  the  last  traces  of  nitric  acid,  and  the  residue  dissolved  at  a 
gentle  heat  in  a  definite  volume  of  dilute  hydrochloric  acid.  The  operator 
in  this  manner  obtains  an  idea  of  the  strength  of  the  solution. 

Pure  platinum  chloride  may  be  recovered  from  the  precipitates  with 
the  alkaline  chlorides  which  are  obtained  in  analytical  work  by  boiling 
them  with  a  solution  of  sodium  carbonate  and  alcohol  :  washing  the 
precipitated  platinum  with  hot  water  by  decantation  and  finally  with 
hot  hydrochloric  acid.  The  spongy  metal  is  then  dissolved  in  aqua 
regia  (5HC1  :  iHNO3),  the  solution  filtered,  and  evaporated  to  dryness 
with  hydrochloric  acid  as  above. 


TREATMENT    OF    « SILVER    RESIDUES.' 

The  mixed  silver  salts,  associated  with  metallic  silver,  which  accumu- 
late in  the  course  of  analytical  work,  may  be  conveniently  reduced, 
after  washing  and  drying,  by  heating  to  fusion  with  a  mixture  of  sodium 
and  potassium  carbonates  in  an  earthen  or  unglazed  porcelain  crucible. 
The  button  of  metallic  silver  is  washed  with  boiling  water,  and  dis- 
solved in  nitric  acid,  arid  the  solution  of  the  nitrate  evaporated  to 
dryness. 


INDEX    OF    SEPARATIONS. 


ALUMINA  from  calcium,   iron,  magnesia, 
potash,  and  soda,  101 

—  from  iron   in  presence   of  phosphoric 
acid,  177,  220,  246 

—  from  iron,  101,  102,  184,  246,  248 

—  from  copper  and  bismuth,  213 

—  from  lead  and  tin,  108 

—  from  tin,  107,  108,  109 

—  from  bismuth,  copper,  and  lead,  213 
BARIUM  from  calcium,  92 

—  from  strontium,  178 

BISMUTH  from  antimony,  arsenic,  copper, 
and  lead,  109,  213,  272 

—  from  silver  and  cadmium,  261 
CALCIUM  from  magnesium,  88,  185 

—  from  magnesium  and  alkalies,  99,  180, 
185 

—  from  phosphoric  acid,  347 

COBALT  from  arsenic,  antimony,  bismuth, 
iron,  zinc,  &c.  214 

—  from  nickel,  214,  275 

—  from  zinc,  214,  244 

COPPER  from  antimony,  arsenic,  and  tin, 

212 

—  from  bismuth  and  lead,  109,  212,  272 

—  from  lead,  tin,  zinc,  and  iron,  103 

—  from  zinc  and  nickel,  106 
GOLD  from  silver  and  copper,  286 
IRON  from  alumina,  101,  102,  184,  246 

in  presence  of  phosphoric  acid, 

177,  217,  247 

—  from  manganese,  177,  214,  247 

—  from  nickel  and  cobalt,  214,  247 

—  from  chromium,  247,  268 

—  from  zinc,  105,  214,  244 

LEAD  from  arsenic  and  antimony,  108 


LEAD  from  bismuth,  copper,  tin,  iron,  and 
zinc,  103,  108,  213 

—  from  copper,  antimony,  iron,  zinc,  &c. 
258 

—  from  silver,  256 

MAGNESIUM  from  alkalies,  102,  180,  185 

—  from  calcium,  88,  185 

—  from  calcium  in  presence  of  phosphoric 
acid,  347 

MANGANESE  from  alumina,  178,  220,  247 

—  from  iron,  177,  214,  247 

—  from  nickel  and  cobalt,  214,  247 

—  from  zinc,  214,  247 

MERCURY   from   silver,    copper,    arsenic, 

antimony,  iron,  and  zinc,  278 
NICKEL  from  antimony,  arsenic,  copper, 

lead,  bismuth,  &c.  214 

—  from  cobalt,  215,  275 

—  from  iron  and  manganese,  214,  247 

—  from  zinc,  106,  214,  247 
POTASSIUM  from  magnesium,  99,  174,  185 

—  from  sodium,  85,  128 

SILVER  from  copper  and  gold,  286 

—  from  lead,  256 

—  from  bismuth  and  cadmium,  261 
SODIUM  from  magnesium,  99,  174,  185 

—  from  potassium,  85,  128 

—  from  bismuth  and  lead,  109 

—  from  copper,  lead,  iron,  and  zinc,  103 

—  from  tungsten,  253 

TUNGSTEN  from  iron  and  manganese,  254 

—  from  tin,  253 

ZINC  from  cobalt,  215,  244 

—  from  iron,  105,  214,  246 

—  from  manganese,  214,  247 

—  from  nickel,  107,  214,  249 


GENERAL    INDEX. 


ACETIC  acid,  determination  of^trength 
of,  142 

Acids  in  combination,   volumetric   deter- 
mination, 146 
Acidimetry,  142 
Air  bath,  39 

Air  in  water,  analysis  of,  327 
Albite.  analysis  of,  102 
Alkalies,  determination  of,  in  glass,  too 

limestone,  178 

manures,  332 

—  plant  ash,  340 
Alkalimetry,  130 

Alkaline  liquids,  action  on  glass,  47 
Alumina,  determination  of,  98 
Ammonia,  determination  of,  in  gas  liquor, 
141 

guano,  141 

by  volumetric  analysis,  140 

by  gravimetric  analysis,  94 

in  water,  293 

—  manures,  340 

—  (albuminoid)  in  water,  297 
Ammonium    chloride  solution,   action  on 

glass,  47 

Analysis  (indirect),  principles  of,  92 
Antimony,  estimation  of,  107,  108 
• —  volumetric  determination  of,  in  tartar- 
emetic,  161 

Arsenic,  determination  of,  in  cobalt  glance, 
274 

—  fahl  ores,  278 

—  iron  ores,  219 

Arsenious  acid,  estimation  of,  by  iodine, 

165 

—  solution,  preparation  of,  194 
Ashes  of  plants,  analysis  of,  340 
Atomic  weights  and  symbols  of  elements, 

369 


BALANCE,  description  of,  3 
—  adjustment  of,  7 

—  conditions  of  stability,  &c.  8 

—  tests  of  efficacy,  13 

—  preservation  of,  15 


Balance-room,  15 

Barium  and  calcium,  separation  of,  92 

—  determination  of,  in  limestone,  177 

—  and  strontium  sulphates,  separation  of, 
177 

—  sulphate,  analysis  of,  91 
Baume's  hydrometer  tables,  374 
Bell  metal,  analysis  of,  103 
Bismuth,  estimation  of,  109,  213 
Black  ash,  analysis  of,  198 

—  composition  of,  199 

Bleaching  powder,  composition  of,  193 

—  valuation  of,  194 
Bone  dust,  analysis  of,  337 
Boulangerite,  analysis  of.  279 
Bournonite,  analysis  of,  279 
Brass,  analysis  of,  103 
Braunite,  analysis  of,  185 
Britannia  metal,  analysis  of,  107 
Bromates,  analysis  of,  165 
Bromine,  determination  of,  in  organic  com- 
pounds, 364 
Bronze,  analysis  of,  103 
Bullion  (silver),  assay  of,  281 
Burette,  graduation  of,  119 

—  Gay  Lussac's,  119 

—  Mohr's,  121 

—  reading  of,  122 


CADMIUM,  determination  of,  as  sul- 
phide,  240 

Calcium    chlorate,    determination    of,    .in 
bleaching  powder,  197 

—  volumetric  determination  of,  by  potas- 
sium permanganate,  152 

Carbon,  determination  of,  in  iron  and  steel, 
229 

Eggertz'  method,  235 

Ullgren's  method,  233 

Weyl's  method,  231 

—  Wohler's  method,  229 

—  dioxide,  gravimetric  determination  of, 
86 

by  volumetric  analysis,  143 

in  aerated  waters,  145 


C  C 


386 


General  Index. 


Carbon  dioxide,  gravimetric  determination 

of,  in  natural  waters,  144 
Carius'  method  for  determination  of  sulphur 

and  phosphorus,  367 
Cement  stone,  analysis  of,  182 
Chlorates,  analysis  of,  165 

—  in  bleaching  powder,  197 
Chlorides,  determination  of,  in  limestone, 

179 

Chlorimetrical  degrees,  198 
Chlorine,  gravimetric  determination  of,  81 

—  volumetric  determination  of,  124 

—  by  iodine  solution,  158 

—  determination  of,  in  organic  compounds, 
364 

—  bleaching  powder,  194 
Chloric  acid,  determination  of,  129 
Chrome  iron  stone,  occurrence  of,  268 

valuation  of,  269 

Chromic  acid,  preparation  of,  234 

Cinnabar,  analysis  of,  288 

Clay,  mechanical  and  chemical  analysis  of, 

Coal,  valuation  of,  289 

—  specific  gravity  of,  291 

Cobalt,  determination  of,  as  metal,  272 
-as  tricobalt  tetroxide,  272 

—  glance,  analysis  of,  273 

Cochineal,  use  of,  in  volumetric  analysis, 

142 

Coins,  silver,  assay  of,  281 
Copper,  gravimetric  determination  of,  74 
—  as  metal,  105,  212 

—  detection  of,  in  waters,  325 

—  ores,  assay  of,  by  Luckow's  process,  207 

—  Steinbeck's  process,  207 

—  oxide,  for  organic  analysis,  preparation 
of,  353 

—  pyrites,  analysis  of,  211 

—  separation  from  gold  and  silver,  286 

—  sulphate,  analysis  of,  71 
purification  of,  70 

Crucibles,  to  remove  silver  chloride  from,  82 

•pVECANTATION,  49 
JL-J     Desiccation,  39 
Desiccator,  37,  38 
Dolomite,  analysis  of,  85 


ELUTRIATION,  36 
Erdmann's  float,  123 
Erlenmeyer's  furnace,  description  of,  349 
Evaporation,  42 
—  of  liquids  containing  gas,  42 
of  high  boiling  point,  43 

FAHL  ore,  analysis  of,  276 
Fat,  determination  of,  in  bone  dust, 
337 

Felspar,  analysis  of,  102 
Ferrous,  ammonium  sulphate,  preparation 

and  analysis  of,  94 
Filter  ash,  determination  of,  51 


Filters,  capacities  of,  59 

—  incineration  of,  51 

—  paper,  analysis  of  ash  of,  50 

—  preparation  of,  51 

—  pump,  6 1 

—  weighed,  use  of,  68 

Filtration,  precautions  to  be  observed  in,  53 
Fire  clay,  composition  and  analysis  of,  182 
Fluorine,  estimation  of,  179 
Funnels,  choice  of,  53 
Fusible  metal,  analysis  of,  109 


GALENA,  analysis  of,  255 
—  determination  of  silver  in,  256 

—  assay  of,  257 

Gases,  from  natural  water,  analysis  of,  327 

—  obtained  in  water  analysis,  examination 
of,  304 

Gay  Lussac's  burette,  119 

—  method  of  silver  assay,  281 
Geissler's  potash  bulbs,  352 
Gelatigenous  matter,  determination  of,  in 

bone  dust,  337 

German  silver,  analysis  of,  106 
Glass,  action  of  solutions  upon,  47 

—  analysis  of,  99 
Gold  assay,  286 

Gold,  silver,  and  copper,  separation  of,  286 
Graphite,  derermination  of,  in  iron,  234 
Gravimetric  analysis,  definition  of,  i 
Guano,  analysis  of,  331 
Gun  metal,  analysis  of,  103 
Gunpowder,  analysis  of,  171 


HARDNESS  of  water,  estimation  of, 
321    . 

Hydrochloric  acid,  estimation  of,  by  gravi- 
metric analysis,  81 

—  estimation  of,  by  volumetric  analysis, 
126 

—  determination  of  amount  of,  required 
to  decompose  manganese  ore,  191 

normal,  preparation  of,  for  volumetric 

analysis,  130 

—  strength  of  aqueous  solutions  of,  377 
Hydrocyanic  acid,  estimation  of,  by  iodine 

solution,  160 


TNCINERATION  of  filters,  51 
L      —  of  plants  for  analysis,  341 
Tlmenite,  analysis  of,  225 
lodates,  analysis  of,  165 
Iodine,  pure,  preparation  of,  156 

—  determination  of,  in  organic  compound*, 
364 

by  sodium  amalgam,  366 

by  alcoholic  silver  nitrate,  366 

—  and  sodium  thiosulphate,  reaction  be- 
tween, 155 

—  solution,  preparation  of,  156 

—  use  of,  in  volumetric  analysis,  155 
Iron,  action  of  acids  upon,  228 


General  Index. 


387 


Iron,  determination  of,  by  potassium  bi- 
chromate, 221 

—  permanganate,  148 

—  iodine    and      sodium    thiosul- 
phate,  1 66 

—  cast,  composition  of,  227 
varieties  of,  227 

—  ores,  analysis  of,  215 
varieties  of,  216 

—  pyrites,  analysis  of,  215 

—  Swedish,  analysis  of,  227 

—  determination  of  slag  in,  249 

—  wrought,  analysis  of,  227 

—  nitrogen  in,  determination  of,  242 

—  by  Ullgren's  method,  242 

—  slags,  analysis  of,  249 

—  estimation  of,  in  natural  water,  326 


KAOLIN,  composition  of,  180 
Kieffer's  solution,  192 
Kupfernickelstein,  analysis  of,  215 


LEAD,  estimation  of,  in  galena,  255 
—  refined,  analysis  of,  258 

—  in  water,  325 

—  volumetric  determination  of,  153 
Liebig's  potash  bulbs,  357 

Lime,  estimation  of,  in  dolomite,  88 
—  felspar,  101 

glass,  99 

manures,  332 

by  volumetric  analysis,  152 

—  in  water,  326 
Limestone,  analysis  of,  174 
Liquids,  organic  analysis  of,  348 
Litmus  solutions,  preparation  of,  136 
Litre  flasks,  graduation  of,  115 


MAGNESIA,   estimation  of,   in   dolo- 
mite, 85 

felspar,  joi 

glass,  99 

manures,  332 

—  water,  326 

—  separation  of,  from  lime,  88 
Manganese,  determination  of,  in  limestones, 

88,  178 

—  iron,  247 
ores,  220 

—  ores,  valuation   of,    by    Fresenius'   and 
Will's  method,  186 

iodine  solution,  185 

—  iron    and    potassium    perman- 
ganate solution,  189 

—  oxalic  acid  and  potassium  per? 
manganate  solution,  189 

—  determination  of  moisture  in,  190 
Manures,  analysis  of,  331 

Mechanical  analysis  of  clay  soils,  &c.  182 

—  division,  35 

Mercury,  estimation  of,  287 
Mercuric  sulphate,  preparation  of,  242 


Mortars,  choice  of,  35 
—  steel  and  agate,  35 


"VTESSLER'S  solution,  preparation,  293 
_L\      Nickel,  determination  of,  106 

—  separation  from  copper  and  zinc,  106 

—  cobalt,  274 

Nickelspeiss,  analysis  of,  279 
Niobic  acid,  detection  of,  254 
Nitre,  analysis  of,  168 

—  estimation  of,  in  gunpowder,  170 
Nitric  acid,  action  on  glass,  47 

estimation  of,  as  ammonia,  96 

by  iodine  solution,  167 

in  nitre,  170 

—  in  water,  315          ^  • 

specific     gravity     ami     strength    of 

aqueous  solution,  378 
Nitrogen,  in  iron,  determination  of,  242 

—  in  manures,  334 

—  determination  of,  by  soda-lime  process, 
334. 

in  organic  analysis,  358 

by  SchifF's  apparatus,  362 

by  Simpson's  process,  359 

in  water  residues,  299 


OLEFINES,  evolution  of,  during  solu- 
tion of  iron,  228 

Organic  analysis,  determination  of  carbon 
and  hydrogen,  348 

—  chlorine,  bromine,  and  iodine,  364 
nitrogen,  358 

phosphorus,  366 

—  sulphur,  366 

Organic  matter,  determination  of,  in  lime- 
stone, 176 

—  in  water,  by  Frankland's  process, 
298 

Orthoclase,  analysis  of,  101 
Oxalic  acid,  determination  of,  by  perman- 
ganic acid,  151 


PEARL-ASH,  analysis  of,  82,  139 
'Phosphorus,  in  iron  ores,  determina- 
tion of,  217 

—  iron,  determination  of,  249 

—  determination  of,  in  organic  compounds 
366 

Phosphoric  acid,  separation  from  iron  and 
alumina,  346 

—  in  plant  ash,  determination  of,  340 

—  water,  determination  of,  327 

—  manures,  determination  of,  328 

—  by  uranium  solution,  328 
Phosphorous  acid,  preparation  of,  289 
Pipettes,  graduation  of,  117 

Pisani's  method  for  determination  of  silver, 
280 

Platinum  crucibles,  precautions  to  be  ob- 
served in  using,  67 

-r-  vessels,  how  to  clean,  67,  68 

—  recovery  of,  from  residues,  345 


CC  2 


388 


General  Index. 


Platinum  tetrachloride,  preparation  of,  381 
Potassium  bichromate,  valuation  of,  163 

use  of,  in  volumetric  analysis,  112 

and    sodium    thiosulphaie,    reaction 

between,  157 

—  carbonate,  valuation  of,  83,  139 

estimation  of,  in  presence  of  sodium 

carbonate,  139 

—  chromate   solution,   preparation   of,  for 
volumetrical  analysis,  126 

—  cyanide,  valuation  of,  160 

—  preparation  of  standard  solution  of, 
206 

—  ferricyanide,  estimation  of,  154 

—  ferrocyanide,  estimation  of,  154 

—  gravimetric  determination  of,  83 

—  hydroxide,  determination  of,  in  presence 
of  carbonic  acid,  138 

—  iodide  paper,  preparation  of,  195 

—  permanganate  solution,  determination  of 
strength  of,  148 

—  use  of,  in  volumetric  analysis,  in, 
148 

—  and  sodium,  indirect  determination  of, 
82,  136 

Potash  alum,  analysis  of,  98 

—  pumice,  preparation  of,  234 

—  solution,  strength  of,  379 
Precipitates,  drying,  65 

—  igniting,  64,  65 
Precipitating  flasks,  50 
Pyrites,  copper,  analysis  of,  212 

—  iron,  analysis  of,  215 

—  in  limestone,  determination  of,  176 
Pyroligneous  acid,  determination  of,  142 
Pyrolusite,  analysis  of,  185 


"P  EICHARDTS  method  of  expelling 

J\.     gases  from  water,  328 

Rider,  use  of,  in  weighing,  23 

Rochelle  salt,  analysis  of,  85 

Rosolic  acid,  use  of,  in  volumetric  analysis, 

MS 
Rutile,  analysis  of,  226 


SAMPLING,  method  of,  34 
Scheelite,  analysis  of,  255 
SchifTs  apparatus  for  measuring  nitrogen 

in  organic  analysis,  36^ 
Sifting,  method  of,  36 
Silica,  estimation  of,  in  clay,  181 

glass,  99 

felspar,  102 

water,  326 

—  plant  ash,  346 

in  limestone,  175 

*» iron  ores,  217 

*> in  copper  ores,  211 

•*-  —  iron,  245 

' galena,  256 

smaltine  and  cobalt  glance,  272 

Silver,  assay,  281 

—  determination   of,  by  Pisani's  method, 
280 


Silver,  separation  from  gold  and  copper,  286 

—  ore,  red,  analysis  of,  279 

—  preparation  of  pure,  124 

—  in  galena,  determination  of,  956 

—  in  solutions,  determination  of,  279 

—  residues,  treatment  of,  382 

—  solutions,  standard,  preparation  of,  125 
Slag,  determination  of,  in  iron,  249 
Smaltine,  analysis  of,  272 

Soap  solution,   preparation  of,   for  water 

analysis,  321 

Soda  ash,  valuation  of,  137 
analysis  of,  199 

—  solution,  preparation  of,  for  volumetric 
analysis,  135 

—  strength  of  aqueous  solutions  of,  380 
Sodium  carbonate,  estimation  of,  in  pre- 
sence of  potassium  carbonate,  1 39 

—  and  potassium,  indirect   estimation  of, 
by  volumetric  analysis,  128 

—  gravimetric  determination  of,  82 

—  chloride,  purification  of,  79 
analysis  of,  79 

—  separation  from  potassium,  85 

1    —  hydroxide,  determination  of,  in  presence 
of  carbonate,  38 

—  salts,  estimation  of,  in  water,  327 

—  thiosulphate,  preparation  of  solution  of, 
158 

—  sulphide,  determination  of,  by  Lestelle's 
method,  203 

Soluble  matter,  determination  of,  in  water, 
315 

Sphene,  analysis  of,  226 

Spiegeleisen,  composition  of,  249 

Sprengel  pump,  use  of,  in  water  analysis, 
301 

Starch  solution,  preparation  of,  for  volu- 
metric analysis,  157 

Steam  bath,  38 

Steel,  analysis  of,  228 

—  composition  of,  229 

—  action  of.acids  upon,  229 
Sulphates,  determination  of,  in  water,  326 

I    Sulphur,  determination  of,  in  iron,  238 

volumetrically,  240 

coal,  290 

copper  pyrites,  211 

iron  ores,  217 

galena,  255 

— •  —  roasted  pyrites,  205 

organic  compounds,  366 

by  Carius'  method,  367 

refined  lead,  265 

Sulphuretted  hydrogen,  estimation  of,  by 

iodine  solution,  159 

in  mineral  waters,  160 

Sulphur  dioxide,  estimation  of,  by  iodine 

solution,  159 
Sulphuric  acid,  action  on  glass,  48 

gravimetric  estimation  of,  77 

volumetric  estimation  of,  135 

separation  of,  in  presence  of  alkalies, 

169 

determination  of,  in  limestone,  178 

in  manures,  332 


General  Index. 


Sulphuric  acid  solution,  normal,  prepara- 
tion of,  133 

strength  of  aqueous  solution,  376 

Superphosphates,  analysis  of,  337 


n^ABLE  furnace,  100 

J.      Tap  cinder,  analysis  of,  250 
Tartar  emetic,  amount  of  antimony  in,  161 
Tetrahedrite,  analysis  of,  276 

—  composition  of,  276 
Tin-crystals,  valuation  of,  162 

—  determination  of,  by  iodine  solution,  162 

—  ore,  assay  of,  252 

—  separation  of,  108,  109 

• — •  and  tungsten,  separation  of,  253 
Titaniferous  iron  ores,  analysis  of,  225 
Titanium,  determination  of,  in  clays,  184 

—  iron  ores,  218,  226 
Titanite,  analysis  of,  226 
Triangle,  68 

Tungsten  and  tin,  separation  of,  253 
Twaddell's  hydrometer,  375 
Type  metal,  analysis  of,  108 


T  TRANIUM  solution,  preparation  of,  for 
v_J      volumetric  analysis,  332 


VAT  waste,  analysis  of,  200 
Vinegar,    valuation   of,  by   Kieffer's 
solution,  192 

Volumetric  analysis,  definition  of,  2 
—  principles  of,  no 

WASH  bottle,  54 
Water,  action  of,  on  glass,  47 
—  bath,  44 


Wash  bath,  Bunsen's,  46 

—  collection  of,  for  analysis,  291 

—  determination  of  albuminoid  ammonia 
in,  297 

ammonia  in,  293 

chlorine  in,  320 

ni  rates  and  nitrites,  315 

—  silica  in,  326 

iron,  326 

lime,  326 

magnesia,  326 

sulphuric  acid,  326 

—  phosphoric  acid,  327 

—  lead  and  copper,  325 

—  sodium  and  potassium,  327 

total  soluble  matter,  315 

hardness,  321 

—  suspended  matter,  293 

—  in  combination,  determination  of,  71 

—  estimation  of  sulphuretted  hydrogen  in, 
159 

—  in  minerals,  estimation  of,  183 

—  in  manures,  estimation  of,  331 
Weighing,  correction  for  displacement  of 

air  in,  32,  381 

—  by  vibrations,  27 

—  operation  of,  20,  21 

—  by  substitution,  21 

—  precautions  in,  25 
Weights,  description,  16 

—  method  of  testing,  18 

—  tarnished,  to  clean,  18 

White  lead,  composition  and  analysis  of, 
267 

—  adulterations  of,  267 

Wolfram,  occurrence  and  analysis  of,  254 


'INC  ores,  assay  of,  250 


PRINTED    BY 

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LONDON 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 
BERKELEY 

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